Solids can often diffuse into a liquid but cannot diffuse into another solid.

This statement is generally true due to the differences in particle mobility in different states of matter.

1. Understanding States of Matter: Solids, liquids, and gases are the three fundamental states of matter. In solids, particles are tightly packed and have very little mobility. In liquids, particles are less tightly packed and can move around more freely. In gases, particles are very loosely packed and have high mobility.

2. Diffusion in Liquids: When a solid is placed in a liquid, the particles of the solid can slowly spread out or "diffuse" into the liquid. This is because the liquid particles have enough energy to move around and make space for the solid particles to fit in. This is why, for example, a sugar cube dissolves in a cup of hot tea.

3. Diffusion in Solids: On the other hand, when a solid is placed in contact with another solid, the particles of the first solid cannot easily diffuse into the second solid. This is because the particles in both solids are tightly packed and have very little mobility. They do not have enough energy to move around and make space for the other solid's particles to fit in. This is why, for example, a sugar cube does not dissolve in a block of ice.

4. Exceptions: There are some exceptions to this rule. For example, in certain conditions, atoms in one solid can diffuse into another solid. This process is called solid-state diffusion and is much slower than diffusion in liquids or gases. It often requires high temperatures to provide the atoms with enough energy to move.