Tardigrade

Tardigrades were microscopic creatures on Earth. They lived in the air and in the water, capable of surviving the coldest temperatures and living almost forever.

To explain them to Clara Oswald, the Eleventh Doctor got her to stare at the TARDIS column for ten seconds, then shut her eyes, count to five, and open them again. All she saw were those "tiny little things that float in front of your eyes when you look at a bright light". She hypothesised that this was just fluid being reflected back, but the Doctor corrected her. (PROSE: Normality)