There are 12 balls. 11 of them are identical. One of them differs in weight from the others. You are given a pair of balances but no weights. What is the least number of weighings in which the odd ball can be sorted out?

Note: Obviously the rocks have to be weighed against one another since you are given no weights.

2
4
5
3

**
This section requires Javascript.
**

You are seeing this because something didn't load right. We suggest you, (a) try
refreshing the page, (b) enabling javascript if it is disabled on your browser and,
finally, (c)
loading the
non-javascript version of this page
. We're sorry about the hassle.

https://en.wikipedia.org/wiki/Balance

puzzle#Thetwelve-coin_problem