One of my favorite early demonstrations is this one on "sewer lice." Basically, you submerge raisins in carbonated water (or white soda) and they will float to the surface and then sink and repeat this all day long. I ask my students to make a guess about whether they think the creatures are alive. Sometimes I even add a backstory about the creatures being discovered in the sewer and that they are edible. You can eat one to really mess with the freshman!
Great discrepant event demonstration. I used to use a Mountain Dew style soda (for the color similarity to urine) and white raisins. I would tell them the 'sewer lice' metabolized the toxins from the liquid waste . After a few minutes of observations by the students about characteristics of life, I would then drink some of the liquid.
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