"Start with a millimetre. Imagine it divided into a thousand equal widths. Each one of those widths is a micron. This is the scale of micro-organisms. A typical paramecium, for instance- a tiny, single celled freshwater creature- is about 2 microns wide (0.002 millimetres) which is really very small. If you wanted to see a paramecium swimming in a drop of water, you would have to enlarge the drop until it was some 12 metres across. However, if you wanted to see the atoms in the same drop, you would have to make the drop 24 kilometres across.
To get down to the scale of atoms, you would need to take each one of those micron slices and shave it into ten-millionth of a millimetre. It is a degree of slenderness way beyond the capacity of our imaginations, but you can get some idea of the proportions if you bear in mind that one atom is to that millimetre line above as the thickness of a sheet of paper is to the height of the Empire State Building." Bryson