Fingers puzzle.

You have 10 fingers,count your fingers from 1-10, 1,2,3,4,5,6,7,8,9,10 now count backwards, 10,9,8,7,6 on one hand plus the other hand 5 fingers so 6+5=11 why?

We used to do that in primary school. It's in the same box as the shared-payment-partial-refund thing used by teachers as an example of apples&oranges thinking.