How do the Incredible Hulk's pants stay on?

When he’s mild mannered Bruce Banner, his slacks fit just fine.

But he can change into the Hulk in matter of seconds, when he is in danger or his heart beats more than 200bpm. He goes from being a normal size (maybe a 38 -40) to hulk-size (BIG waist and other body parts).

So two related questions:

1) when he bulks up (hulks up) how does he not utterly destroy his pants and shirts? They can’t possibly expand to his hulkified body.

2) when he de-hulks and returns to his normal size, how do his pants fit? They were shredded and ripped because of his hulkification. Why do they fit now?

Is spandex that stretchy?