Revealing mistake: When Bob and Ben prepare to gas the ant nest, an ant appears below them. But its legs move too slowly, compared to the speed in which it appears; it rather looks like it is driving across the ground on (hidden) wheels.
Revealing mistake: When the giant ants' nest is taken under fire with bazookas, it can be seen that the back-blast jets are just inserted, e.g. the 'zookas are not actually fired. Also, there is a cut between the 'firing and some of the explosions visible in the same frame.
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.
stiiggy