Why does milk come up when boiling ???

 We all have observed that when we boil milk, after some time milk raises up.

At this point, if we don't switch off the gas, milk overflows.

Ever wondered why ???

Milk is a colloid.  Milk has water, protein and fatty particles in it.

When we boil milk,  the fatty particles get collected in the upper layer of milk thus forming a creamy layer.

This layer blocks the steam which is produced in the bottom of vessel from going out.

But when the pressure built at the bottom is more,  this pressure comes out by raising the milk along with the creamy layer.

If we don't stop this pressure building up,  milk overflows..