Science - monster marshmallows

Marshmallows have a squishy texture because they are filled with tiny air pockets. Gases like air are made of molecules moving freely at high speed and bouncing off any surface they meet. 

As they bounce, the molecules put pressure on the surface. Heating a gas causes the molecules to move faster, increasing the pressure. When you heat a marshmallow each air pocket blows up like a balloon making the marshmallows enormous.