two marbles puzzle

[xela]

A friend of mine was asked this question when he interviewed at Google:

You have two identical marbles and access to a 100 story building with windows that open on each floor. What is the minimum number of trials of dropping a marble from a window necessary to determine the highest floor at which a marble will not break? Describe the the procedure. You may destroy both marbles in the process.

I solved this fairly quickly; my friend, who's smarter than I am, didn't. Of course, I wasn't under interview pressure. Anyway, I'd like to discuss the solution, but first I'd like to give my friends a shot at it. Comments are screened; in a few days I'll post again, discussing the solution. (Comments to this post will remain screened.)

Edit: My friends are a bunch of smart asses. But we all knew that. Minimum number of trials in which you can be certain of finding the highest floor from which a dropped marble will not break, whichever floor it is.