Googol

Words of wisdom are spoken by children at least as often as by scientists. The name "googol" was invented by a child (Dr. Kasner's nine-year-old nephew) who was asked to think up a name for a very big number, namely, 1 with a hundred zeros after it. He was very certain that this number was not infinite, and therefore equally certain that it had to have a name. […] A googolplex is much bigger than a googol, much bigger than a googol times a googol. A googol times a googol would be 1 with 200 zeros, whereas a googolplex is one with a googol of zeros. You will get some idea of the size of this very large but finite number from the fact that there would not be enough room to write it, if you went to the farthest star, touring all the nebulae and putting down zeros every inch of the way.

For example, in considering all the finite state grammars that use seven terminal symbols and seven auxiliary symbols (states), [...] he must test over a googol (10^100) candidates.

If the universe were packed solid with neutrons, say, so there was no empty space anywhere, there would still only be about 10¹²⁸ particles in it, quite a bit more than a googol but trivially small compared to a googolplex.