Solids are not always denser than water. In fact, there are many solids that are less dense than water, such as ice, cork, and wood. The density of a substance is determined by its mass per unit volume. Solids are generally denser than water because they have a higher mass per unit volume. This is because solids have a more tightly packed structure than liquids, so they can fit more mass into a smaller volume.

However, there are some exceptions to this rule. For example, ice is less dense than water because the hydrogen bonds between water molecules cause them to form a more open structure when they freeze. This means that ice has a lower mass per unit volume than water, so it floats on top of water.

Similarly, cork and wood are both less dense than water because they contain a lot of air pockets. These air pockets reduce the mass of the cork and wood, so they can float on top of water.