Shabbos 23:5

We may not drum, dance, or clap our hands on Shabbos, as a preventive measure against coming to repair a musical instrument, though one may clap with the backs of his hands. One is not permitted to swim on Shabbos, which is a preventive measure against the possibility of making a raft. One may swim in a pool in his courtyard so long as it has a raised rim that will keep the water from flowing out. This serves to differentiate between a pool and the sea.

Shabbos 23:6

One may not cut a reed on Shabbos because doing so resembles preparing a utensil. If a reed was cut before Shabbos, even though it has not been trimmed, it may be inserted into the hole in a barrel on Shabbos without concern that one might come to trim it. One may not put a myrtle leaf or something similar in the opening of a barrel for the wine to flow through because doing so resembles making a faucet. One may not break a shard of ceramic or tear a piece of paper on Shabbos because doing so resembles preparing utensils.