Say you have a balloon full of a type of gas, it is completely isolate from everything else. Let the temperature of the gas be 20 degrees (or any temperature, it doesn't matter)
Now, the ballon is blown up because the molecule of the gas is constantly colliding with the "skin" of the balloon, stretching it into a big, blown up balloon.
But as each time the molecules of the gas collide with the "skin" of the balloon and other gas molecules, it loses some kinetic energy, and will then begin to slow down.
Since temperature of a gas is dependent on the kinetic energy, it will cool down.
But from school, I remember learning that temperature of an isolated system will remain constant forever if heat is not transferred.
So why would the gas cool down? and why or why not?