You recently participated in this. Just heads up that I have created a new event to make up for it.
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Hi :)
Happy holidays
https://www.steamgifts.com/giveaway/OrttU/

Heh, I've reached the point of throwing my hands up. I tried childish humor (i.e. what kids leave in public pools). I tried various tools you can use to fill a pool. I tried pool accessories and games. The repeated red response has finished me.

If I'm not mistaken, you are thinking of "pools" in terms of human-made pools. The answer (and hint) are referring to "pools" found in nature. The answer itself, is a natural occurrence. Let me know if you could use more hints. :-)