Imagine you have four "see-through" boxes, all the same size, and you fill each
of them up with a different material.
Try to guess which material has the least mass, which has
the most, and where the other two fit in between. (Mass is closely related to weight. Here on earth, for all
practical purposes, it's the same thing.) Test your prediction by placing the
materials two at a time on the balance below, and when you're ready to pick the one
with the least mass, click on the box of your choice above.