I'm not a physics expert, but this is more or less my understanding of what happens: Heating an object means that the individual atoms that make up the object move around faster. As they move around faster they collide with each other more often and more energetically. This makes the atoms recoil from each other and therefore in general, all the atoms in the obejct end up beign further apart from each other. This makes the object expand. As it exands, it becomes less dense and therefore less heavy. A classic example is a hot air balloon. The air inside the ballon is heated and expands, becomes less dense and therefore lighter than the air outside the balloon eventually resulting in the ballon raising off the ground - esssentially the lighter air inside the balloon floats up through the cold denser / heavier air outside the balloon - same way as a plastic bottle full of air will float in water - air is lighter / less dense than water - hot air is lighter / less dense than cold air.