In a spin from Separatix's answer, my guess is water (or any other known fluid, but there's none better known than water by most people). Water behaves wildly different at this scale : insects routinely can transport bubbles of water without container by just holding it.

I don't know the physical details, but I doubt you could get a container of 1mm or smaller to hold water in the same way something at our scale does : just due to capilarity, the water will "climb" the walls of any container for a small distance. But at this scale, the distance will be very very big. And puddles of water can very easily be way bigger than a few mm. You will very easily see there's something wrong when you see water holding out in a giant bubble instead of spreading on the floor.

Freezing or boiling water also do the trick : snow flakes are of rougly equal size and you would be able to see them way bigger than usual, and bubbles in boiling water will be huge.

In a spin from Separatix's answer, my guess is water (or any other known fluid, but there's none better known than water by most people). Water behaves wildly different at this scale : insects routinely can transport bubbles of water without container by just holding it.

I don't know the physical details, but I doubt you could get a container of 1mm or smaller to hold water in the same way something at our scale does : just due to capilarity, the water will "climb" the walls of any container for a small distance. But at this scale, the distance will be very very big. And puddles of water can very easily be way bigger than a few mm. You will very easily see there's something wrong when you see water holding out in a giant bubble instead of spreading on the floor.

Freezing water also do the trick : snow flakes are of rougly equal size and you would be able to see them way bigger than usual.