If somebody is approaching you about a simple question that you know is impossible to answer, you want to respond with a demonstration of your expertise. 

Let them know the uncertainty is justified; show historical evidence that what they are asking is difficult to predict.

Look at the evidence or circumstances and let the person know what it might suggest. Ask some clarifying questions. Make an informed estimate.

Make a good plan to help improve certainty and make sure the person feels assured.

If you’re an expert, make sure that you know how to communicate like one.