What about solids ? Do solids move around like liquids and gases ?

No, gases move freely all over the place, liquids move more freely but in a more contained area, and solids the molecules are rigid, at the most the movement is small vibration as the molecules put pressure on one another to hold their form and stay in place.

if the concentration of products is decreased, the equilibrium will shift toward the products). add an inert gas (one that is not involved in the reaction) to the constant-volume reaction mixture: this will increase the total pressure of the system, but will have no effect on the equilibrium condition.

explanation:

dynamic equilibrium is the answer.