We're Always Thinking Wasserbazooka

Screwing an empty soda bottle onto the back instead of buying refills is genius, and it actually seals tight with no leaks. My kids emptied two liters in about ten minutes and just swapped in another bottle.

My kid grabbed this at the beach and spent two hours refilling it from our cooler bottles instead of running to the tap. It screws right onto a standard soda bottle, so no leaks or fiddling once it's tight. Bright colors made it easy to spot when it got dropped in the sand.

Screwing it onto a regular soda bottle instead of refilling a tiny tank is genuinely clever. My son carried it around the pool for two hours and it never leaked once.