To play hide and seek, you hide behind a pole or a car, not behind a glass window, why?

On the basis of which common property would you like to group these objects, and why? 

Book, sand, a metal key, sponge, leaf, cork, an eraser, and an iron nail.

Solve the given puzzle.

Down

1. A gas which is insoluble in water.

2. A substance which does not allow electricity to pass through it.

4. Objects through which we are able to see, but not clearly.

7. Objects through which we are not able to see.

Across

3. A substance which can be compressed.

5. A substance which cannot be compressed.

6. A substance which can dissolve in water.

8. Object through which we are able to see clearly.

9. A substance which cannot dissolve in water.