Answer to previous riddle: There is just as much milk in the tea as there is tea in the milk. As long as the volume of liquid in both cups remain the same at the end (i.e. half the cup), then whatever milk isn't in the milk cup must be in the tea cup. Likewise with the tea.
Very counter-intuitive. Which is why I love this one. The next one comes from a series of similar riddles that involve pouring exact amounts of liquid given odd-sized jug. I was first introduced to these in grade 3!
Given 9L and 5L jugs, give me exactly 6L of water. Their volumes are measured at the brim. The jugs are oddly shaped, but so you cannot estimate any proportion of the jug (i.e. you can't "eyeball" where half is). You cannot put any marks on the jug or otherwise keep track of where water levels were.