I've always just put an ice cube in a ziplock bag. If it isn't square, that means it's melted & refrozen. The marble trick seems like it's probably more sensi

I've always just put an ice cube in a ziplock bag. If it isn't square, that means it's melted & refrozen. The marble trick seems like it's probably more sensitive, but I also don't have marbles and film canisters laying around.