This might be a good activity for outside, or somewhere with plenty of room. Find a large ball and give your baby time to explore it, they might want to hold it or push it away. Give your baby some other objects, this could be another ball or one of their toys. Give them time to look at and explore these, then show them how if you hit the ball with another object the ball moves. Do this gently at first and pause to give your baby time to watch and think about what has happened. Move the ball close to them and see if they want to have a try themselves.