Final answer:
The pressure inside the balloon increases due to a decrease in volume caused by squeezing the sides inward.
Step-by-step explanation:
The pressure inside the balloon increases when the sides of the balloon are squeezed inward due to a decrease in volume. According to Boyle's Law, as the volume of a gas decreases, its pressure increases.
Similarly, when the sides of the balloon are squeezed inward, the gas particles are forced closer together, resulting in a decrease in volume and an increase in pressure.
For example, if you partially fill an airtight syringe with air and slowly push in the plunger, the gas in the syringe is compressed into a smaller volume, and its pressure increases.