Counting on your fingers is useful on many occations. Most of us probably count up to five per hand.
Counting to 5 is easy, because most of us have five fingers. Try it now: Put your index finger of your right hand on the thumb of your left. That's 1. Go to the finger next to it. That's your index finger. 2. Then your middle finger. 3. The one that is likely to be a little bit shorter than your middle finger, is your ring finger. That's number 4. The shortest one - and furthest removed from your thumb - is your pinky. That's number 5. Congratulations - that was counting to 5 on one hand.
Beyond the First Five
With a little bit more dexterity, we can easily count up to 12. Or 16. Or 31. On one hand!
Just think about it: Fingers have folds or creases in them. So why not count on them? Counting to 12, apparently, is quite normal in some Asian countries. You just use your thumb to indicate the first joint, second joint, or tip of a finger. That's three on one finger. Times four = 12. Hurray.
Counting to 16 is very similar, but instead of only counting the three points mentioned in counting to 12 in the paragraph above, you count on the base of the finger as well. Base + first joint + second joint + tip = 4. Times four = 16.
Counting to 31 - If you are particularly dexterous, you can use your fingers to count in binary. You have 5 fingers = 5 bits. thumb is 1, index is 2, middle is 4, ring is 8 and pinky is 16. 1+2+4+8+16 should be 31, unless this early hour has me swooning with incomprehension (although it can be verified by knowing that the sixth bit is 32s, so five bits should be 31).
Try it now: Count to 31 on your fingers! If you find it a bit tricky to hold, say, only your ring finger extended, place your hand on a table, palms down, and fold fingers in and out, you should be able to do it just fine.
00001 (thumb extended) = 1
00010 (index finger)= 2
00011 (index plus thumb)= 2+1 = 3
01000 (ring finger) = 8
01001 (ring plus thumb) = 8+1 = 9
01010 (ring plus index) = 8+2 =10
11011 (all but middle) = 16+8+2+1 = 29
11110 (all but thumb) = 16+8+4+2 = 30
11111 (whole hand) = 16+8+4+2+1 =31
Of course, unless you are a concert pianist, you may find this tiring, not to mention physically difficult. But it can certainly be done. You could use the other hand to help extend your fingers, or use a table edge or something. If you feel brave, you could even count up to 1023 (11 bits minus one) using two hands.
Counting to 100 on two hands is also quite possible, by assigning the number 5 to your thumb of your left hand, and 1 to each of your fingers. Thumb of your right hand is 50, all the other digits are 10 each. This method is known as Chisenbop.
Who said human beings are limited?