A person dives into a swimming pool that doesn't happen to be filled.
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Maintenance worker, to diver: Are you going to dive into the pool?
Maintenance worker: (putting up "Pool Closed for Repairs" sign while diver dives) Good. At least that's not empty.
--MAD Magazine: Snappy Answers to Stupid QuestionsYou know the gag. A person makes a masterful dive into a swimming pool, discovering too late that the pool doesn't have water in it. Physical trauma (and possibly hilarity) ensues. It's fairly popular in comedy, and typically Played for Laughs, although more dramatic examples do exist. A common subtrope of High Dive Hijinks.