[eru]

Happened to me regularly when returning from summer holidays.

[ars]

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.

[eru]

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

[ars]

Vegetable oil will oxidize and turn into a sticky mess.

[eru]

OK, thanks.