"What’s this thing I found on the floor of my bathroom? Between 1 and 2 cm in size and the black ring is a magnet that can rotate within its axis."
A: "It looks like part of the diffuser in the faucet fixture, if you unscrew the round part under the faucet spout there are often little pieces like these that either help regulate flow or restrict, regulate the flow."
"credits www.reddit.com"
I like these "What is it?" posts.
It confuses the Americans.