When the bottle is placed into the bowl of cold water, the air inside the bottle is cooled. This causes the air molecules to move slower and closer together. The air contracts and causes the outside air to rush in. This pulls the balloon inside the bottle and inflates it. 

Molecules within a substance, including air, are always in motion. The energy of this motion is called kinetic energy. Temperature is simply a measure of the average kinetic energy of a substance. The faster the speed of the molecules, the higher the temperature. The lower the temperature of a substance, the slower the speed of the molecules. Theoretically, if a substance is cooled to absolute zero (-273° C), all molecular motion would stop. This is the lowest temperature possible. Researchers have not been able to reach this temperature yet in a lab, but have come very close! As the temperature of a substance, such as air, increases, the molecules move further apart (expand) and the air becomes less dense. There become fewer molecules in a given space. As the temperature of a substance decreases, the molecules become closer together (contract) and the air becomes more dense. The amount of molecules in a given space will increase. 

Heat is defined as energy being transferred from one substance to another due to a temperature difference between the them. In our bottle experiment, this is done primarily by a process called convection. Convection is defined as the transfer of heat by a mass movement of a fluid. The air in our atmosphere behaves like a fluid. Another method of heat transfer, called conduction, occurs when heat is transferred directly from molecule to molecule. A good example of conduction is when you leave a spoon in a pot of boiling liquid while cooking on a stove. That spoon can get really hot! The warm plastic of our bottle heats the air molecules touching the inside of the bottle by conduction. Since air is a very poor conductor of heat, only an extremely shallow layer of air along bottle surface is heated. The molecules in this very shallow layer gain extra energy, expand, and this layer becomes less dense. Bubbles of warm air rise and transfer heat and energy to the molecules surrounding them by convection. Cooler, less dense air sinks down to the bottle surface and the process is repeated. The air in the bottle behaves similar to the way the ground heats the air on a sunny day. Radiation from the sun heats the ground and then the ground in turn heats the air mainly by convection. 

The reason the balloon is pulled inside the bottle and inflated also has something to do with air pressure. Temperature and air pressure are closely related. In this experiment, we are focusing on temperature and heat transfer. Air pressure will be explained in another experiment coming soon.