Fire ants behave like a fluid and like solid. In the first video we see a penny falling through a column of ants. This is analogous to dropping a steel ball through honey to measure how slowly it falls.
The second video shows the solid-like behavior of fire ant aggregations. The solid like nature can be see two ways: 1) the ants are maintaining a shape, and 2) when compressed with the petri dish the ants return to their original shape.
For more see our article in Nature Materials