Activity page (with video) from the Royal Institution.

You’ll find this activity all over the internet and in loads of science activity books, and this is a particularly nice film both illustrating it and exploring how you might use it as a family project. There’s plenty to discuss around around why some things float and others sink, and hence the concept of density.

This is another film from the Royal Institution’s ExpeRimental series, all of which are worth checking out.

Updated March 2026: links now point to the most appropriate equivalent pages at the Royal Institution’s current website.