Float maybe for a time, but not for long....I wish folks wouldn't post those pics, misleading.
What kind of cereal containers? Am picturing cardboard boxes. Not cereal buyer, so <shrugs>
This what I've found to work the best for cooling chickens,
better than ice in waterer, frozen foods, etc:
I give a dose of Sav-a-Chick electrolytes/vitamins about once a week during heat waves. Iit really seems to help....started this after they saved a heat stroked hen once.
...and BIG(9x14x2") chunks of ice last all day for wading, sitting, and sipping.
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Make space in your freezer:
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I absolutely second the Sav a Chick and the ice, plus deep shade is an absolute must. I live in AZ and give Sav a Chick every few days when it's hot here. We aren't just hot during the day, our overnight lows can be over 90* for quite a while too.