Picture a helium balloon on a string. I hold one end of the string, and the balloon rises until the string is taut. With the balloon hovering at its location, and with tension on the string, I cut the string near the bottom. The balloon will rise, taking the string with it. So, the question is, what rises first, the balloon, or the bottom of the string? The balloon can't rise while the string is holding it down. But the string can't rise unless the balloon pulls it up. Assuming there is no elasticity in the string, it seems that neither the balloon nor the string can rise. I guess this is similar to the idea of pushing one block against the other, Block A into Block B. Block A can't move while Block B occupies that space, and Block B can't move unless something else applies a force to it. So, how can to surfaces in contact apply for to each other, with one object displacing the other, if they can't occupy the same location at the same time?