Monkeys in the Sack

A guy walking down a street one afternoon passes an old man sitting on the side of the road with a large sack.
The younger guy says to the old man:

- Watcha got in the sack?

The old man responds:

- I got some monkeys in that there sack.

The younger man asks:

- If I guess how many monkeys you got in the sack, can I keep one?

The old man replies:

- Son, if you guess how many monkeys I got in this sack, I'll give you both of 'em!