Liquid Helium exhibits zero viscosity, meaning it can flow up the side of a beaker and escape. It also conducts heat perfectly.
Some fluids, called non-Newtonian fluids, change their viscosity based on the force applied to them. A strong blow will make them act as a solid.
Plasmas are by far the most common fluid in the universe. They are also the most common state of matter. This is because stars and the thin matter between solar systems are plasmas.
Some materials, such as carbon dioxide, cannot exists as a liquid under normal pressures. It can exist as ice, but will sublimate into gas as soon as the temperature is high enough.
Diamonds are the hardest natural material, but synthetic diamond are harder still.
Under extreme conditions, such as earthquakes, solid ground momentarily behaves as a fluid.
All metals exhibit a liquid state, and Thorium has the highest liquid range of temperature of any element.
Osmium is the densest element, but by no means the densest form of matter. The hugely compressed stuff inside neutron stars is much denser: a teaspoon would weigh as much as the earth.
Water is one of the most common molecules in the universe, but it often exists in scattered pockets in the form of ice.
Sound travels much faster in a solid than in a gas, which is why you can hear a train coming at great distance by putting your ear to the rail.
Carbon can form molecules theoretically infinite in length. This property is exploited in industry to create a vast variety of specialty chemicals.
Water achieves it's greatest density in liquid form, at around 4 degrees Celsius. This means the water at the bottom of the sea can never freeze, due to the compression caused by the water column above. It can still be heated and carried upwards by convection.
Like ice in water, solid Bismuth will float in liquid Bismuth.