When I interviewed at Google in 2006, one of the interviewers asked me to write code for solving a particular problem. I replied along the lines of "ok I need to think about this, it's not obvious how to solve it" and started thinking. Time ticked away. Five minutes. Ten minutes. Fifteen minutes. The interviewer started reminding me that he needed me to write some code and if there wasn't anything for him to copy dow…
But also, it's a basic interview technique to explain your thinking out loud.