I never judge a candidate in the software industry based on brainteasers (there are a lot of pros and cons anyway),
but this one is really funny:
There are two equal containers, one of water and one of wine.
You take ladle of wine and put it in the water, later on you take the same ladle of the water-wine mixture and put it into the wine container.
Is there more wine in the water container, or more water in the wine?
A complete answer here: http://www.donaldsauter.com/wine.htm