Had a good talk with my 6th grade period about density; I really didn’t expect to talk for the whole period about it, but it went really well. We talked about why you (hopefully) float in water. Then defined density and measured the density of a rock. This helped the kids understand why they sink.
But the fun part was towards the end. I brought up buoyancy, and one student made the connection to the Titanic. That as it filled with water its density changed. Eventually its density tipped over to greater than 1.0 and it sunk. That was a great connection. Then I talked about boats that are supposed to sink: submarines. How they can change their density by using the ballast tanks. The kids were pretty interested the whole time.