Biology Experiment that isnt too long!?

I need a biology science fair project. I need an experiment that will take one day max! This science fair project is due in three days and i dont have a topic yet. i had one but that backfired. I have litmus paper so maybe a topic on pH would be good. Anyways your suggestions will be grately appreciated. p.s im in Gr.12 so i need a high school level topic.