The direct sunlight will heat up the molecules making up the gases in the balloon.
Kinetic energy resulting from the heating of these particles will make them become randomized. They then begin to move more haphazardly colliding vigorously with themselves and the wall of balloon.
According to Charles's law "the volume of a fixed mass of gas is directly proportional to its temperature provided pressure remains constant" .
The sunlight heats the balloon and causes a rise in temperature. This will make the gas volume to increase and the balloon expands.
The nervous system helps all the parts of the body to communicate with each other. It also reacts to changes both outside and inside the body. The nervous system uses both electrical and chemical means to send and receive messages.
