This myth comes from the Ancient Greeks, for whom the only way to do this would be to sit on the beach and count each grain one at a time! This would take a very long time and they would not be able to write down or describe such a big number.
The Greeks had the word ‘myriad’, which meant 10,000 (written as 104), but to most people this was such a large number that they thought of it as meaning ‘uncountable’. Greater numbers such as millions (106), billions (109), trillions (1012) and googols (10100) were not yet in use. The last term googol was first used by Milton Sirotta, 9, in 1938. A googolplex then is a one followed by a googol of zeroes!
Archimedes (287–212BC), in his book The Sand Reckoner, described a way to count the sand grains by making bigger and bigger units. The following is an example. If a beach is 500m long, 10m wide and 2m deep, then the volume of sand in the beach is 10,000m3 or 1010cm3. If 125 grains of wheat make up one cubic cm, the beach would contain 125 x 1010 = 1.25 x 1012 grains of wheat. If 20 grains of sand take up the same volume as a grain of wheat, then there are 20 x 1.25 x 1012 = 2.5 x 1013 grains of sand in this beach. This amounts to 25 trillions of grains of sand in this particular beach. Similarly, one can calculate the number of sand grains on every beach in the world and add them all up!
Scientists have even estimated the total number of particles in the whole universe. Tony Padilla, a physics professor at the University of Nottingham, obtained the estimate 3.28 x 1080, which is even less than one googol.