Why do you think structures that are no longer used shrink in size? There is no need to expend the energy to make it full- size, because it isn't needed. Smaller structures help organisms move faster.

## Step 1<br />The question is asking why structures that are no longer used shrink in size. This is a biological phenomenon that can be explained by the principles of energy conservation and the need for efficient movement.<br /><br />## Step 2<br />The first reason is that there is no need to expend energy to make a structure full-size if it is not needed. This is based on the principle of energy conservation, which states that energy cannot be created or destroyed, only transferred or transformed. In biological systems, energy is a precious resource, and organisms strive to use it as efficiently as possible. If a structure is not needed, it would be wasteful to produce it in full size, as it would require energy that could be used for other purposes.<br /><br />## Step 3<br />The second reason is that smaller structures help organisms move faster. This is because smaller structures are lighter, which reduces the force required to move them. This is based on the principle of Newton's second law of motion, which states that the force required to move an object is equal to the mass of the object times its acceleration. By reducing the mass of the object, the force required to move it is also reduced, which can speed up movement.