Ask HN: 5th Grade Science Fair Project Ideas

Last week someone asked about good college science fair ideas so I thought this might be a good place to ask about 5th grade science fair ideas. The neighbor boy has asked me (being the neighborhood geek) for a good science fair idea. He's a bright kid and could tackle a somewhat complex project. The complication: it's due Wednesday, he asked his mother to call me about it two weeks ago and she didn't - until one hour ago. So, any good ideas that can be done on such short notice?