You find out from your doctor that you are dehydrated and need an IV. Use the arrows to choose the IV that you want (the left is pure water and the right is salt water). When nothing is happening, press "SPACE" to move on. When you play the game, use the arrow keys to move and to collect all of the salt water drops without catching any pure water.
This has been one of the most fun school assignments in 7th grade and I wish more were on scratch.