A Design Review of the Christmas Booby Traps from Home Alone
Home Alone is a classic Christmas Season movie. It’s also the only movie I’ve ever watched back to back in its entirety. On VHS. The 5 minutes waiting for it to rewind was just enough to get more popcorn and snacks before watching it again. I’m both proud and a little sickened that my parents let me watch it twice in a row.
For those unfamiliar with the movie, Kevin, a young boy, is accidentally left home alone at Christmas time while his family leaves the country on an international vacation. In the meantime, two thieves are stealing valuable assets from homes in Kevin’s neighborhood and they have targeted Kevin’s home, which they thought was empty for the holidays and would, therefore, be an easy target. Kevin takes it upon himself to defend his house with all sorts of clever booby traps that an eight-year-old might design.
I was 10 years old when this movie came out and the same age as the actor who played Kevin. Ten years old is also the prime age for planning pranks and setting up simple booby traps for siblings and parents. Setting up objects to fall on a person when they entered the room and putting food coloring in toilets was so last year. But with this movie, I now had a treasure trove of ideas to try out on my “enemies.”
But, the downside of growing up and trying things out is that you quickly realize most of those booby trap ideas won’t work or are simply too dangerous.
My Christmas present to all of you is a quick design review and critique of Kevin’s booby traps from Home Alone with a few comments and a highly subjective star rating out of 5. (If you haven’t seen “Home Alone”, take some time and place it in the Christmas movie line-up between “It’s A Wonderful Life” and “A Christmas Story.”).
And now the ratings of the Home Alone booby traps:
1. Freezing water onto the sidewalk or steps. This is simple but very effective. Still, it requires a low enough air and ground temperature and little or no solar radiation sources (i.e. it works better at night). The freezing temperature can be lowered with salt so if the area has ice melt, it won’t work as well. An even more dangerous condition is when that sheet of ice is smooth, dark, and covered with a small skiff of snow. And ice on steps is even more nefarious. I’ve been the victim of my own attempts at making sheets of black ice in Canada. Very dangerous. Low cost. High performance. But don’t ever do this. 4 out of 5 stars.
2. Burning hot doorknob. Kevin heats up the front doorknob with a BBQ starter. We don’t know how hot it gets in the movie but it apparently is hot enough for second degree burns when one of the thieves tries to open the door. The knob is in fact glowing red. In reality, there might be other indicators like smoke or burn marks around the knob as well (although the movie doesn’t have those). It does show him holding on much too long but that’s just for cinematic exaggeration. As a design critique of this prank, I think it would fail. 1 out of 5.
3. Iron falling onto head. When a thief pulls on a fake light switch, he pulls an iron down the laundry shoot onto his own head. With a 2 kg iron (including water), and a 10 m fall (around 3 stories), the kinetic energy of the iron at impact would 196 Nm. Assuming that full energy is absorbed in the deformation of the thief’s face over 2 mm, the average impact force could be 196/0.002 = 98000 N (and the peak impact force could be even higher!). I’ll let you disagree with my assumptions and decide what would happen, but you should know that the best boxers can punch up to 5000 N. I realize this prank is a variation on the classic “dropping an object on a person” trick, but there would be a bunch of noise and the natural instinct would be to recoil back soon enough. I don’t buy it. 1.5 out of 5 stars.
4. Blowtorching one’s head after opening door. This is where I started thinking Kevin is a little obsessed with fire and heat. After opening the back door a thief holds his head below the flame much longer than necessary severely burning his head. He should have been physically done after this one. I predict third-degree burns and an immediate trip to the hospital. As a booby trap, this one would quickly cause any person to draw back and thus would work as a serious deterrence. Beyond the obvious joke, this would cause major damage to any living organism. 2 out of 5.
5. Walking into plastic wrap with glue. There are lots of variations on this one. Especially in poorly lit environments although this is inside with lights on. It’s also not as physically damaging as the others but it’s often the source of a good laugh. Also, the glue would have to be slow drying to maintain a tackiness level long enough. The humor is intensified when the thief is covered with feathers triggered by a nearby fan. Very doable and entertaining. 4 out of 5.
6. Tripping on wire. Pretty basic. But it also never gets old. Any prankster has this one on their resume from their youth and returns to it time and time again. The wire also needs to be strong and taut enough for tripping the person (for the feathers, Kevin used a simple string trigger). Since, in the movie, the thief does a flip after running into the tripwire it must be relatively strong. It looks about a quarter inch in diameter, which if stranded would be strong enough, but since it’s tied to a baluster, the baluster might give at the tension forces required for a flip. Still, it might work with a less dramatic result. 3 out of 5 stars.
7. Swinging paint can into face. Kevin launches two paint cans from the top floor tied to a rope and using the pendulum action hits the thieves in the face who are on the stairs. As an engineer, there are some potential geometry and kinetic energy problems with this set-up but it could work in theory in Kevin throws the cans at the right angle. The damage could be high with a direct hit but that’s unlikely if you can’t see from up above. Yes, the movie has two direct hits for maximum slap-stick comedy potential. 3 out of 5 stars.
8. Stepping on Micro-Machines. This happens all the time in my home. In my experience, it’s usually with Lego but the result is the same. Absolute pain in the foot. Hoping up and down. Complete debilitation for a minute or two. I have been both the receiver and accidental giver (when I failed to clean up after my Lego session) of this special Christmas gift. 4 out of 5 stars.
9. BB gunshots. I don’t know if this really qualifies as a booby trap. Kevin is pretty much shooting pellets or ball bearings at the thieves with his own hand. Maybe I laughed as a kid, but shooting anything at a person is no laughing matter. I guess we all grow up eventually and thinking about right now, it’s not even that creative. 1 out of 5.
10. Sticky tar on steps with exposed nail. This prank would be terrible to clean up but generally sticky pranks can be effective and sometimes safe. Superglue used responsibly can be fun and educational. The thief’s shoes and then socks stick to the tar before he ultimately steps barefoot onto a protruding nail. That’s painful. I can see it happening. 3 out of 5.
11. Stepping on broken Christmas tree ornaments. Although this is easy to set-up, the chances of it working are almost zero since anyone breaking into a house probably has shoes on. Yes, the thief just barely lost their socks and shoes (see above) but that’s just the clever plot sequencing. Overall, I rate this one lower as it won’t work in the real world. 2 out of 5 stars.
12. Cutting a rope while someone tries to cross over it to a treehouse. Again, this one is funny but real-world execution would be next to impossible. The thieves also hold on after Kevin has cut the temporary zip line with a hedge clipper. This makes them swing back into the wall of the house like another pendulum while they hold on to the rope. Without assistive friction devices, the grip force required at which they’d have to hold themselves up during the swing would be at least their mass (say 80 kg) times the acceleration due to gravity (9.81 m/s^2), or near 800 N. Since many experiments suggest the maximum human grip strength would be 500 N or less, this result is highly unlikely (although, Spiderman seems to do it with some additional help). Furthermore, the angles, lengths, and geometry don’t add up here either, but I suppose it’s entertainment. 2 out of 5 stars.
There are few more quasi-pranks involving a tarantula and crowbars, but the above list has the vast majority presented in the movie. For those readers that have seen Home Alone, do you agree? How would you rate or rank these booby traps in terms of effectiveness? Cost? Reliability? Other design dimensions?
Oh yeah. It goes without saying, but don’t try any of these at home. Call the police instead. I’m grateful for the humor and the stunt people that made the movie entertaining, but these pranks really won’t work at all to keep your home safe at Christmas time or at any other time of the year.
Merry Christmas!
