Two kids are playing around with their drinks at lunch. One has 200ml of milk and the other has 200ml of chocolate milk. They scoop 20ml of chocolate milk into the milk glass, then they scoop 20ml of whatever is in the milk glass into the chocolate milk glass.
When they’re done, is there more chocolate milk in the milk glass or more milk in the chocolate milk glass?
Solution
Neither, there will be an equal amount of chocolate milk in the milk glass and milk in the chocolate milk glass!
The simple way to think about it: both glasses start and end with the same total amount of liquid, so any chocolate milk in the milk glass must have been replaced by milk in the chocolate milk glass.