What happens to the particles in a basketball when it becomes flat?

PV=nRT

n (moles of gas), R (constant value of .0821) are constants in this equation. So PressureXvolume is proportional to temperature. As temperature is decreased, so is the pressure and the volume, ie the ball becomes flat. This is just to help you visualize, the actual answer is that when the temperature decreases, the energy of the gas particles decreases as well. and because kinetic energy is related to velocity, the speed of the molecules slow down and are not hitting the sides of the ball from the inside as frequent and the ball appears flat.

A decrease in temperature leads to a decrease in the movement of molecules in a system (example, when water freezes, the water molecules slow down enough to align and bond to form ice).

Also, from an ideal gas law perspective (PV = nRT), if there is a decrease in temperature at constant volume, pressure must also decrease (while the volume is the same, the molecules are condensing / closer together in space (like water vapor condensing to form liquid water).

Since heat is the energy that molecules/atoms/particles have, then cold is where they aren't moving as much. Therefore, the molecules come closer together, not taking up as much space as they did before.

Rather than bouncing off the walls of the ball, they are now sort of relaxed, not moving as much. They become more dense.

Ever wonder why you feel cold water better than warm water?

Not just because it's freaking cold, but cold water is much more packed and dense, where warm water molecules are more spread out.