This is how to prevent water boiling over

Why does this occur

A watched pot never boils, but an unwatched pot can go from boiling to overflowing pretty quickly. But there is a way to keep your pot from boiling over.

When you put a pot of water on, put a wooden spoon over the top of the pot. 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.

Why does it work 

First, bubbles are unstable forms. When something hydrophobic (i.e. unable to absorb water) punctures the surface of a bubble, it subsides.

A dry wooden spoon destabilizes the bubbles when they come into contact with its water-repelling surface. This makes the boiling water retreat.

Second, bubbles and foam created from boiling water are filled with steam (aka water in the form of a gas or vapor). If the bubble touches something whose temperature is below 100°C (212°F), the steam will condense (change back into a liquid) and break the surface of the bubble.