This video from the Royal Society of Chemistry (RSC), aimed at primary teachers, demonstrates an experiment which may be used when learning about gases and forces.
A milk bottle is filled with water then a ping pong ball placed on the top. When up-turned the water stays in the bottle as the large surface area of the ping pong ball means that the force acting on it is greater than the pull of gravity on the water. Following this demonstration, children could be asked to try and explain what they have observed.
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