Cups of all colors and sizes (I think half a stack of nesting cups would be perfect)
Gather materials. Fill dish bin with about 2 inches of warm water. Have at it! Don't leave your little one alone with this as it poses a drowning hazard. Have towels on hand to wipe up spilled water. Obviously everything in the vicinity is going to get soaked so don't put your child in the outfit she is wearing to Aunt Mod's 93rd birthday party that night.