Two simple but interesting questions to refresh your O Level physics knowledge:
1. There is a man and a piece of rock the size of a basket ball in a little boat that is floating in a swimming pool. The man throws the rock into the water. Does the water level in the swimming pool rise or fall?
2. A deflated balloon is place on one side of a balance and a balloon filled with air (same composition as the ambient atmosphere) is placed on the other side. If both the balloons have identicle mass before one of them is inflated, which side does the balance tip towards?