As we kick-off the Christmas season, reruns of holiday movie classics like Home Alone begin to play on our TVs. Despite the movie’s warm family values and comic relief, some of the traps that Kevin McCallister (Macaulay Culkin) sets for the Wet Bandits Harry (Joe Pesci) and Marv (Daniel Stern) appear to present the possibility of serious injury. How dangerous are Kevin’s pranks, really?

We shouldn’t look too critically at the science that appears in movies, especially light-hearted movies like Home Alone. That notwithstanding, we can certainly apply scientific principles to some of the stunts that young Kevin pulls on two would-be burglars who intend to rob his house on Christmas Eve. Let’s consider two of them.

Red-Hot Door Knob
To prevent the crooks from getting in, Kevin hangs a red-hot charcoal starter over the inside of the knob on his front door. When Harry grabs the outside of the doorknob, his hand gets so hot that it begins to smoke.

The color of a glowing object depends on its temperature, and for the most part, this relationship is the same for all materials. A heated object starts to glow a faint, dark red at about 900^oF. As the temperature goes up the color gets brighter red, then orange, then eventually white. From the shade of red that Kevin’s charcoal starter is glowing, we can deduce that its surface temperature is about 1,500^oF.

How hot is 1,500^oF? The National Institute of Standards and Technology says that human skin is instantly destroyed when exposed to a temperature of 162^oF. So touching a 1,500^oF doorknob for a second would cause serious injury. Could it cause burglar Harry’s hand to burst into flame? The ignition temperature—the temperature at which something begins to combust—is about 600^oF for wood and about 850^oF for coal. So 1,500^oF is, well, dang hot! We should suspect that Harry’s hand would do more than just smoke.

The Falling Clothes Iron and Paint Cans
As Harry sneaks through the basement, he looks up to see a falling clothes iron that strikes him square in the head. Later, as the two men climb the stairs, cans of paint swing down at the ends of ropes and, again, smack the burglars in the head. Would the resulting injuries be worse than simply making the men a little groggy, as depicted in the movie?

Objects accelerate as they fall. The iron and those cans of paint would be moving 20 to 25 miles an hour upon impact. The extent of any injury is related to this speed and the weight of the iron or the cans.

The severity of the injury also depends on the duration of the contact. Longer contact time, less effect and less injury. This is the reason we have air bags in cars. In an accident, an airbag extends the time a person’s body is in contact with the steering wheel or dashboard, lessening the blow. Because the skull gives just a little during an impact—the bones deform slightly and then spring back— the duration of the impacts of the iron and cans are extended just a bit.

Taking good guesses for the weight of the iron and paint cans, and the time they are in contact with the men’s foreheads, we can estimate the force they exert to be around 100 pounds. For a forehead, that’s a lot.

It’s not just the amount of force that determines the pain or severity of injury. Pressure, which takes into account the area over which the force acts, also plays a role. Imagine walking on a beach of fine pebbles. This doesn’t hurt because your weight is spread out over an area the size of your feet. But step on a single pebble… that would hurt! Same force but smaller area means higher pressure and more pain.

Pushing with 100 pounds of force on the relatively small area of a forehead results in a relatively high pressure. However, the part of your skull behind your forehead is one of the strongest bones in your body. It would likely withstand the force and pressure exerted by the iron and the falling cans. So although the burglars in the movie Home Alone will feel like they’ve had their bell rung after getting smacked by those falling objects, they likely won’t experience permanent injury.

So it’s not bad news for Marv and Harry in terms of getting smacked in the noggin. But Harry—you shouldn’t have touched that doorknob!

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Questions to ponder:

Assuming you enjoyed the Home Alone movies, would you enjoy them less if you knew that in real life some of the pranks would be really dangerous?