Why do we count in groups of tens ? Is counting in tens the best way to count ?
Very interesting questions to consider ! Clearly, the presense of ten digits on our hands must largely explain why human count in tens because there are some clear dis-advantages with the decimal system. For example, it is difficult to count in tens using mechnical and/or electronic devices, and, in fact, computers work largely using binary (counting in twos); octal (eights) and hexadecimal (sixteens) counting systems are also used by computers and machines. "Ten" is also not as useful as duodecimal ("counting in twelves") for dividing quantities (i.e. 12 can be divided by 1, 2, 3, 4, 6 and 12, whilst 10 can only be divided by 1, 2, 5 and10). Old timers in the civilised world, and citizens of the the USA, will tell you the foot/inches are more useful than meters/centimeters for practical measurements because of this ease of division associated with duodecimal. I don't think there is any desire to move from a 24 hour clock to a 20 hour clock -I woudn't like to lose four hours of sleep ! - because of the very practical nature of the current system.
Thus, in part, we count in tens because of an accident of evolution (or the will of a god according to some). It is fun to imagine a world where we have eight fingers and thumbs and not ten. My dream of an eight finger universe can found in an earlier blog (2/1/09) entry for though inclined to entertain wild thoughts or interested in counting systems.
Note: Some societies count in fives (one handed) and others don't consider numbers past two ...... one, two and many. I'm not aware of any societies that count in 20s, which is a logical extension of counting with all available digits, but imagine how difficult remembering your times tables would be in such a society !