You can't really have a range of baby toys without a ball! Balls are so fundamental to babies development and they never seem to tire of watching the ball roll away from them and back towards them.

Balls probably come in the babies list along with cardboard boxes and keys of the most wanted items. There's all sorts of science of course, that developing hand eye co-ordination early can help with sports later on, but much more important for us than that is the joy of watching a baby transfixed and then so excited by a ball.