A function f from A to B is called onto if for all b in B there is an a in Asuch that f (a) = b. All elements in B are used.

All fingers on left hand corresponds to other fingers on right hand.

A function f from A to B is called one-to-one (or 1-1) if whenever f (a) = f (b) then a = b. No element of B is the image of more than one element in A.

all five fingers on left hand are different but similar to some fingers on right hand side

and atleast one finger corresponds to other finger on both sides.