from the book, "Our Fascinating Earth"
It is difficult to describe in meaningful terms just how infinitely small atoms and molecules really are. A single ounce of water contains approximately a trillion molecules of H20 (the chemical formula for water). If each water molecule in that ounce were enlarged to the size of a golf ball, all the ocean basins in the world could not contain them. Nor is the molecule the smallest building block in the division of matter, for it is itself made up of combinations of much smaller atoms.
There are many facts to dazzle the mind of those who are able to think small. The smaller a living creature is, the thicker will appear the liquid in which it must swim. To make this more comprehensible, the motion of a bacterium in ordinary water is comparable to that of a human swimming in liquid asphalt! However, since the bacterium does not know the difference, it is quite comfortable and adapts easily to its viscous environment.
It is difficult also for the nonscientist to maintain a frame of reference for the minute size of bacteria. They are the tiniest of free—living cells, so small that in one drop of water there may be as many as a million individual bacterial cells. Even with this vast population there is still enough space for them to move about quite freely in this single drop of water.