You’d think it would be pretty difficult to get a 110-pound iron anvil to float on top of a liquid, but it’s definitely possible with the right substance. In this clip from Cody’s Lab, he shows how a tub filled with shiny liquid mercury does the trick. The much higher density of the mercury is why this experiment works.
Beyond its liquid properties, the other cool thing about mercury is that it’s magnetic. Roobert33 shows how electromagnets can be used to change the direction that mercury flows in. It would be cool to build an Escher-style fountain that flows uphill with this method.