Nobody cares until they have an empty box in their head
Why are you presenting and talk about A when the audience wants to hear about B? Well, for example, because A is the only thing you know how to talk about?…
If the audience of undergrads is looking for a lab rotation or research experience, don’t tell them about textbook molecular biology you are working on. Tell them how your day looks, and how the lab space looks
If the audience wants to know whether your microscope is good for their problems, don’t give lecture on physics of light. Show images of samples and experiments that come out.
If the audience wants to go home and be left alone… present the absolute minimum amount that will satisfy your supervisor, and let everyone free. Seriously, everyone loves when meetings end early
Q. What if you get to Q&A sooner, but there aren’t any questions?
A. Everyone goes to get a beer! Seriously, if there are no questions it means all questions were answered or the audience isn’t interested. In both cases it’s time to go.