In the episode titled "The Jerk", House and his team are dealing with a speed-chess prodigy who is a jerk to everyone.

About 20 minutes in, House is speaking with a man who got sunburned but is wondering why he has spots on his chest that remain unburned. House fills a syringe with water and sprays the man to make his son laugh. The syringe is about 1/3 full.

House offers to sell the kid his syringe for the $1.41 in the kid's pocket. The kid jumps up to pull his money out. As the sunburned man starts to ask how House knew how much money the kid has, we can easily see that the syringe is now about 1/2 full.