Question

when we boil water it do not come up but when we boil milk why does it come up....
i don't know which subject it belong maybe chemistry....

Answer 1

Milk rises when boiled because about 80% of it is water. During heating and boiling process fats, proteins and all the minerals which are lighter than milk gets separated and forms a different layer on the top known as cream. 
Now as heat is constantly supplied some of the water from milk changes into vapours. Before the formation of top layer which takes some time water just exits the vessel in form of steam, but after the formation of creamy layer it has no path to escape from rest of milk. With the increase in time heat increases expanding water vapours, which then start to push top layer and thus forms foam on the top and through there is no space remain it comes up.

Answer 2

hey!!
bcoz milk is more dark. and during boiling the milk something layer is fomed.. due to this....
hope it helps...