Trivia: When Ben Peterson is killed by a giant ant, he emits a high-pitched, half-choked death scream. This scream has been used in many other movies ever since: in Small Soldiers (when Brick Bazooka loses his legs); in Star Wars - A New Hope, when Luke shoots a stormtrooper in the shaft-swinging scene; in Lord Of The Rings - The Two Towers, when an Elf warrior is thrown off the Helm's Deep wall; and in Lord Of The Rings - Return of The King, when a Gondor soldier fleeing from Osgilliath is snatched up and dumped by a Nazgul steed. I guess that makes it the most used death scream in movie history.
Trivia: Leonard Nimoy has a uncredited appearance as an army sergeant at the hospital.
Answer: You fill the canvas bag with water, put it somewhere where the air can blow through, and the evaporation of the water seeping through the canvas creates cool water inside.