A watched pot never boils, but an unwatched saucepan can go from boiling to overflowing pretty quickly. Redditor slokotkov shows us one way to keep it from boiling over.
When you put water on to boil, put a wooden spoon over the top of the saucepan. If it starts to boil up too high, the spoon will pop the bubbles and keep it from boiling over as quickly, meaning you have more time to get back and turn the heat down.
Note that this could warp a wooden spoon, so do it with a utensil you don't want as much. Alternatively, you could use something metal, but you'd have to remove it with an oven mitt because it will become very hot (unlike wood, which won't transfer as much heat). Hit the link to read more.