Every parent's nightmare: The science behind stepping barefoot on Lego

Ever stepped on a rogue block of Lego in the middle of the night? You know the feeling we are talking about. Photo / Getty Images

Tuesday night, in an act of movie promotion-slash-celebrity torture, comedian and actor Will Arnett walked barefoot across a pile of Lego bricks. The pit was styled in the fashion of firewalkers' embers, to highlight these toys' sole purpose: pain.

Arnett, who voices Batman in the upcoming The Lego Batman Movie, hustled through the pit on Jimmy Kimmel Live. The physical sensation of walking on the bricks, as thousands of parental feet have felt before him, was clear; Arnett's face wavered between the type of grimace perfected by Clint Eastwood and the numb smile perfected by Grimace the hamburger alien.

In the decades since the first Lego brick was manufactured in 1949, the toys have earned a widespread reputation as a scourge of bare feet. In 2015, a French advertising agency unveiled a pair of Lego-branded slippers, specifically tailored to shield feet from the prickly bricks. The unpleasantness stems from the combination of biology and materials science - what happens when a sensitive appendage stomps on an icon of Danish durability.

Phrases like "Mind over matter," as Kimmel encouraged Arnett, may be of little consolation when the matter involved is so strong and pointy. In March, the American Chemical Society produced a video on the materials behind the "soul-crushing" phenomenon. The bricks are constructed of ABS plastic. The plastic is a polymer chain made up of three different molecules: acrylonitrile, which gives the bricks strength; butadiene, which helps the toys resist deformation; and styrene, for that reflective Lego luster.

The plastic imbues the toys with impressive material qualities. A small and square Lego brick, 2-by-2 stub, can withstand a force of up to 4,240 Newtons, according to tests performed by Ian Johnston, an applied mathematician at Britain's Open University. At the behest of a BBC program, in 2012 Johnston squashed a few of the blocks with a hydraulic tester.

Lego pieces underwent material failure at an average of 4,240 Newtons. This was equal to 950 pounds of force, or the force on a brick at the base of an impossibly tall 375,000 Lego brick tower, the BBC noted. The toys did not snap but instead compressed into cubes that the BBC likened to "warm Camembert."

Enter the sensitive human foot. There are an estimated 100,000 to 200,000 sensory nerves, called exteroceptors, in the bottom of each foot. These nerves inform our brains when we are walking over, say, fresh-cut grass or a sandy beach, so we can adjust our gait accordingly. The nerves, particularly the pain-transmitting neurons known as nociceptors, also inform us when we have encountered a small toy left out on the carpet.

When we step down, our feet do not produce the necessary force to squish a Lego brick into cheese. The brick, remaining rigid, will respond with a force in kind. There are high and excruciating pressures involved. If the average adult American male, weighing 195 pounds, stands atop the smallest square Lego brick with one foot, the resulting pressure will be roughly 550 pounds per square inch.

That is roughly equal to the pressure 1,250 feet below the surface of the ocean. (It's a rough estimate, as writer Karl Smallwood noted, because it ignores the fact that a foot will connect with the floor around the brick - decreasing the pressure - but will also be in motion, increasing the forces exerted.) Even if you're the Dark Knight, that massive amount of pressure stings.