This is a fun powerpoint that outlines the concept of density in kid-friendly language and fun pictures. It is jam packed with diagrams and jokes to keep the kids engaged. It goes on the assumption that students have a very basic understanding of the particle theory of matter, but does review the concept.
Included in the powerpoint is an experiment (very simple) that you can either do as a demonstration, or in small groups. I've used it both ways and its very effective.
Also included are several discussion questions that you can "pause and ponder" while you are going through it to gauge your students' understanding.
This powerpoint is ideally suited for the middle grades (6-8), but could easily be used as an enhancement in younger grades, or remedial in older grades.