>>>Hi everybody,
>>>
>>>This is a puzzle, which could be asked on a interview. I could not find a best answer.
>>>
>>>There is a 100 floor building. You have two glass balls. You need to find out the floor number, starting from which these balls will broke by minimal number of steps.
>>>
>>
>>What is definition of a step in this case?
>
>I think, one dropping of a ball is a step.

Here's an idea. Split floors in N groups of M floors in each. Go up M floors at time and drop the first ball until it brakes. Go down to the privious drop floor + 1 and drop the second ball on each floor moving up until it brakes. The number of attemts in worst case would be

maxnum = N + M - 1.
M = 100/N
maxnum = N + 100/N - 1

All you have to do is to find minimum for this function. I guess it woud be N=10 and maxnum = 10 + 10 - 1 = 19.