Introduced later in the movie, a yellow rubber tennis ball will play a role in the stories of both Jack and Danny. Kubrick will attempt to hide the importance of the ball in the 'making of' documentary filmed by his daughter and aired on the BBC. In the documentary Jack Nicholson takes credit for coming up with the tennis ball, having improvised the idea on set.

ULLMAN I'm afraid you're not going to do too well here unless you've brought your own supplies. We always remove all the booze from the premises when we shut down: that reduces the insurance that we normally have to carry.
JACK We don't drink.
ULLMAN Well then, you're in luck.