The pressure of gas is produced by the collision of gas molecules with the wall of an object. So with the increase of height, there is less and less air, and there is less and less collision between gas molecules and objects per unit area, so the atmospheric pressure gradually decreases, that is, the pressure outside the balloon gradually decreases. However, the number of gas molecules in the balloon has not changed. In order to maintain the balance between internal and external pressures, balloons will continue to expand to increase the contact area between internal gas molecules and objects. If the balloon inflates to a certain extent, it will certainly explode.