A Riddle!

Here is a riddle that my friend told me. I couldn't figure it out, and when he told me what the answer was, I said, "Aww! I can't believe I didn't get that!" Here it is...

A man is walking in a forest when he comes across a black box. He wants to know what is in it, so he tries to open it. He can't because there is no latch or opening. He happens to have a sledgehammer, so he hits it with the sledgehammer. It doesn't break. He shakes the box and listens for anything. He doesn't hear anything. He smells the box. It doesn't smell like anything. Then, he IMMEDIATELY KNOWS what is in the box. HOW AND WHY DOES HE KNOW WHAT IS IN THE BOX?
-It is indestructible
-It is smell-proof
-It is sound-proof
-There is something in the box
-In the box, there is air, but something else
You may ask yes or no questions.