In the classic set-up:
Before your students arrive, you'll pour some Mountain Dew into a glass (another light colored soda would work as well, but there's just something about that neon yellow color of Mountain Dew... ) and add a handful of raisins.
The raisins should start traveling up and down in the soda.
When the students arrive, you show them your sewer bugs, making up a story about how you acquired them and so on. At the end of the story, you drink the bugs (remember, you know it's just soda and raisins) - disgusting your students and forever imprinting yourself on their brains!
As a student activity:
Provide your students with the supplies. Let them set it up and observe carefully to see if they can determine how the "bugs" are traveling.
The explanation:
Raisins have lots of nice bumps and creases to which the carbon dioxide bubbles (found in the soda) can adhere. The carbon dioxide bubbles decrease the density of the raisin, allowing it to rise to the surface. When the raisin reaches the surface of the soda, the bubbles pop. The density of the raisin increases and it drops. Carbon dioxide bubbles once again adhere to the raisin and the cycle continues.
