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The traditional U-trap can fail if abandoned or isn't used regularly too. The water eventually evaporates.

Happened to me regularly when returning from summer holidays.

To prevent it, put a small amount of mineral oil (baby oil) in the trap.

It will float on top and prevent the water from evaporating.

I guess vegetable oil should also work? By the way, I heard of the same trick for keeping water in your vases.

Vegetable oil will oxidize and turn into a sticky mess.

OK, thanks.