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Hi. I am a new teacher who will be beginning a unit on photosynthesis with 7th graders. I am looking for a good "hook" or discrepant event to introduce the topic. We will be doing a btb and chromatography lab with out studies. Thanks!

It's not only related to photosynthesis, but http://www.growingthenextgeneration.com/just-for-kids-games.html has some nice interactive games for students in a student friendly version. You could do this as a review or as a preview of what is to come and then debrief with them about what they learned. You would need internet access though and enough computers.

There are a couple of Page Keeley probes on this topic, too. You can access the book chapters at these two places:
Light and Dark
and this one is by Page Keeley, Francis Eberle, and Joyce Tugel
Plants in the Dark and Light

Carolyn, thank you for the excellent suggestions about Page Keeley's probes; they are designed for teachers to identify misconceptions through discrepant events and enable teachers to learn how to interview students to assess their understanding and knowledge in clear cut ways.

Adah: I also do that lab with the elodea sprigs. I love to see the kids expressions when I take out the test tube from the dark and they seem so puzzled with the color of the water. Then several "light bulbs" turn on and they figure out that plants also go through cellular respiration. This is something even adults don't realize that plants do. People usually associate plants with photosynthesis but they forget that plants need energy at night too....cellular respiration. After this discussion, many kids go "oh...." That lab is one of my favorite labs to do....very easy to set up.