Go into the changing room next door.

You decide to go into the changing room next to Jo's, with a mind to try and peek at her as she undresses. You look around for an excuse to go in, and see a rack of shirts a few yards away. You walk over, grab one and walk in. the cubicle is small, with a mirror, hook and small bench in it.

Hanging the shirt up, you look around for a way to see into next door's changing room. You start making plans for looking over the top with the bench, but suddenly notice something strange about the wall that separates you and Jo. Right in the middle of the wall at about waist height is a very large piece of chewing gum. Ordinarily, this would not be odd, but the size of it makes you think. As you inspect closer, you see that the gum is pinched, as if it has been stuck to the wall and then someone has tried to pull it off again.

You decide to pull the chewing gum off. As you do so, a round hole is revealed, about an inch-and-a-half in diameter. The missing wall is still stuck to the chewing gum. It takes you a few seconds to realise what this hole is. It's a glory hole! You smile at your good fortune, then try to decide what to do with it. On the one hand, you could just stick to the plan and look through the hole, but then, this IS a glory hole...