A good way for children to start to notice the properties of objects is by experimenting with what they can do with things.
The next time your child is playing with a ball, notice with him how it can bounce and roll. What do you think it is about a ball that makes it good for rolling?
Try rolling other things like a block or an empty plastic bottle. Do they roll as well? What do you think it is about a ball that makes it so good for rolling?