The part next to the ceiling is the warmest, because the heated air always ascends.
Why does a soap-bubble ascend?
Because it is filled with heated air from the lungs, and is, therefore, lighter than the air around it.
Why do balloons ascend?
Because they are filled with a gas lighter than the air around them.
Why does a chestnut split open when roasting?
Because the air in it is expanded by the heat, so that it bursts the shell.
Why does the chestnut not split open when a hole is made in the shell?
Because the air can then escape as it expands, without bursting the shell.
What becomes of the air in an apple, when the apple is roasted?