Martin uses water displacement

Here's a description of the contestant's attempt. Someone's probably working on collecting some images to illustrate it.

Martin leaves the lab and returns with a plastic storage container full of water, on top of a wheeled sled. He has Mark submerge his head in the water, and collects the displaced water with a towel. He is then able to weigh the towel to determine the increase in its weight, to estimate the weight of Mark's head (which he puts at about 3kg). He repeats this process with one of Mark's feet in a bowl of water, determining that it weighs about 1.1kg. His total estimate is therefore 5.2kg, and he earns fourth place.

(Written by Karl Craven)