Add one egg yolk and shake. This is a colloid. You might put it on a sandwich. Float some coarse-ground black pepper on water. Ask students why the pepper floats.
Density is the most common answer.
Add a few drops of alcohol to the water and the pepper sinks. Galaxy Gold, a goldenrod colored printing paper. A base turns it red. An acid turns it yellow. Paint a picture or message on the paper with vinegar and let it dry.