What is a good, cost-efficient way to measure conductivity?

Question

What is a good, cost-efficient way to measure conductivity?
It's for the science fair project of a 5th grader that I'm helping.
He needs to be able to measure the conductivity and get a number out of it that he can analyze. (No checking the brightness of a light bulb or the speed of a small motor.)
Thanks.

Answer 1

If you want to measure conductivity, you can get an ohm-meter, which indicates resistance to current flow. Of course, he would have to realize the bigger the numbers he obtained, the poorer the conductor he would have.

There may be conductivity meters around, which measure "mho" (thats real), the reciprocal of ohms.