I use this lab at the beginning of the school year, when we're reviewing measurement. However, it is equallly apprpopriate for the study of osmosis. (As fate would have it, I first did this lab with my 7th graders who go on to study life science, including osmosis. When we got to osmosis, they made the connection back to our measurement study. It was great, and I've never considered doing it any other way, or with any of my other classes).
The procedure is simple enough....
Each student gets a gummy bear*. The gummy bear gets measured thoroughly: length, width, height and mass. Volume and subsequently density can be determined.
The gummy bear then spends a night in a cup of water.
When the students return the next day, the bears get measured once more (after students get over the shock of seeing their newly enlarged gummy bear).
Conclusions are drawn.
In my experience, this lab leads to all kinds of questions for further experimentation... What if I place my gummy bear in Coke/tea/milk/etc? What if I leave my gummy bear in the water for 2 days? What if I allow my newly enlarged gummy bear to sit out for a day? What if I use a gummy worm instead of a gummy bear? If you have the time and resources, it's a great opportunity for students to design their own experimental process and carry it out.
*I tried to use a gummy worm one time, when I had some at home. It didn't work, it completely fell apart. Fortunately, it was just me playing around at home. For that reason, always do a test run on the gummy bears you plan to use with your students. You really want a gummy product that's going to hold up, at least for the initial experiment.