Explain your thinking. What definition, rule, or reasoning did you use to decide

whether something is a solid?

A Solid is one of the three main states of matter. By definition a solid is something whose molecules are so densely packed together that it allows it to keep its shape. Therefore if the item has a definite shape that does not change on it's own it is a solid. As opposed to a liquid which will take and fill the shape of it's container.