Spot the Ball

One of these is a real regulation size tennis ball and one of these is not.
The real regulation size tennis ball is not very good and was only cheap.

I bought it to help with my back problems.
If you roll around on a tennis ball it can help your back apparently.
The other non regulation size tennis ball would really not help with a back problem.

Anyway this blip is a game, can you guess which is the real regulation tennis ball and which is not.