prototype is and how it works in the event that the interviewer is proficient in that language as well.
Most good interviewers will choose a problem that is difficult and one that they feel the candidate may not be be able to solve. The reason for this is the interviewer wants to see that you're able to calmly and confidently work through a problem in order to solve it by asking questions and verbally talking through your thought process. It's neat if you're able to solve some brain teaser right off the top of your head, but most on the job problems will require troubleshooting in order to solve, and that's the most important skill for you to demonstrate. Additionally, good teams want to make sure you are a good cultural fit as well.
It's totally possible that they ask you to solve some silly brain teaser that doesn't really demonstrate your ability to perform well on the team and they don't ask you back. I chalk those off as jobs I wouldn't have enjoyed working in anyways.