When matter behaves like a fluid with a viscosity of zero, it is said to have entered a state of superfluidity.  Viscosity is a measure of how resistive a liquid is to change, the simple example being that things like honey and maple syrup have higher viscocities than water.  Those rare substances with zero viscosity display some very strange properties (see the video).

The first superfluid discovered, and the one that is most easily viewed, is liquid helium, cooled so that it is very close to absolute zero.  The discovery of superfluidity was not made here at Illinois (it was made by a team of Soviet scientists back in 1937), however, decades later in 2003, Illinois professor Anthony Leggett would be awarded the Nobel Prize in physics for his years of work in helping to explain how superfluids act at a quantum level.