You have a 4 oz glass and a 9 oz glass. You have an endless supply of water. You can fill and dump either glass, as many times as you want. What is the fewest amount of steps needed to measure 6 oz of water, using these two glasses only? Outline those steps.

If you have a third container to put partial amounts in as you solve this, it's easy.
If you are constrained to just two containers, it's impossible.
(I can't stand the way teachers are so sloppy wording these puzzles.)

So let's say you have another container.
You fill the 9 oz glass and use it to fill the 4 oz glass. What's left in the 9 oz glass, and what's next?

Sounds like homework...

Glass of Water Puzzler | Car Talk