I have mastered the art of sounding like I know what I'm talking about even when I have no clue at all. It has helped me make many conversations with strangers and also can make me look far more intelligent than I am.

Here are my tips:

1. You probably have to know a little about what you're talking about but when I say a 'little' I mean a very small amount. 
2. Listen to the other person speaking and nod every so often- let them speak for as long as possible and try to pick up on anything they're speaking about.
3. If this is happening in a lesson and a teacher has picked on you and you weren't listening, just stall for time by repeating the question and then try and make something up until they give up and ask someone else.
4. Use hand gestures to make it seem like you are passionate about the subject.
5. Excuse yourself to go to the bathroom and google it on your phone.
6. Try and use the fanciest words you know.
7. If you don't know what to say next, try and divert the attention to something else until you can think of something to say.
8. Literally just let the other person speak for a majority of the time as it means you have less chance of slipping up.