Your question is Random Tweet Reservoir Sampling. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
There is a stream of incoming tweets, and you have a computer with only enough memory to hold one tweet. How do you return a random tweet, where each tweet has a 1/n chance of being returned? For example, if 5 tweets have come in, then there should be a 1/5 chance of returning any one of those 5 tweets.