When you put ice in a plastic bottle and shake it, the ice will start to break apart due to the friction and pressure created by the shaking motion. This could cause the ice to start melting, depending on the temperature of the surrounding environment. The cold from the ice can cause the plastic bottle to contract slightly. If the bottle is sealed tightly, the pressure could increase as the ice melts, potentially causing the bottle to deform or even burst if the pressure becomes too high. If the bottle isn’t sealed, the water will leak out as the ice melts.