"First, think of walking through falling snow. The air seems to be full of flakes, and you can't fit between them, but the air is also mostly empty.
You can walk through the snowflakes because you can easily push them aside. But, now picture that each of those flakes is rigidly held to its neighbors by wires. Then, you could no longer move the flakes aside, and you couldn't take a walk.
In solid materials the nuclei of the atoms are like the flakes, and are held together, or bonded, by the electrons. You can't walk through the walls because your atoms can't move aside for the wall's atoms, and the wall's atoms can't move aside for yours.
If you try to take a walk in a swimming pool, though, the water's molecules are not so strongly bonded together. You can walk through it because you can move the water's molecules aside. But, your body's molecules are tightly bonded to each other, so they don't get pushed aside by the water."
- Gary D