H2O

Imagine you are sitting with your feet in a pool (filled with liquid water), watching the steam (gaseous water) rise from the spa nearby as you sip a drink from your glass filled with ice cubes (solidified water). Not many other chemical substances can exist in all three states of matter within the same temperature range. In fact, water it is the only substance which naturally occurs in all types. This is why H2O (commonly called water) is one of the most interesting chemical compounds in the universe, as well as being essential for life! (We have never found any organism that doesn't contain H2O)  Apart from being the most abundant compound on the Earth's surface, it has many other unique properties such as it's polar covalent bonds.