Before your students arrive, unwrap a Snickers candy bar and a 3 Musketeers candy bar (any size is acceptable).
Have a large, transparent container filled with water set up in the front of the classroom.
Drop the two, seemingly identical, candy bars into the water.
One floats, one sinks.
Nuts are dense. The Snickers, packed with peanuts, sinks to the bottom, while the nut-less 3 Musketeers floats.
You could test your students’ understanding of density by asking them to predict what would happen with other size 3 Musketeers bars. For example, would a king size candy bar float or sink? Some might think it will sink because it’s bigger, but remember, density is an intrinsic property, it doesn’t depend upon size.