Mercury

Mercury is a type of metal found in liquid form in the overworld. It can be made solid by placing cold blocks near it (Ice, liquid nitrogen, etc). It can be picked up in a bucket, or in block form it can be mined with a stone pickaxe or above. However, if left out for too long, the block will melt (even inside chests; in that case it will spill out on the gound aroud your chest). To stop this from happening, you can leave cold blocks near it or place it inside a refrigerator.