Why Hulk's Underwear Doesn't Tear Off - Video

Why don't Bruce Banner's underwear tear off when he transforms into the Hulk?

Marvel canon offers vague explanations involving "unstable molecules" or stretchy purple pants from the 2008 Incredible Hulk film. The real answer is materials science.

Bruce Banner transforms from approximately 5'9" and 150 pounds to over 8 feet and 1,200 pounds. That is roughly a 2 to 3 times size increase across his entire body. Whatever fabric he is wearing has to handle that level of expansion without structural failure.

Here is the breakdown by fabric type:

Cotton

Tensile strength: approximately 287 to 597 megapascals (MPa) Elongation at break: 3 to 7 percent
Cotton has reasonable tensile strength but extremely poor elasticity. Any fabric that can only stretch 3 to 7 percent of its original length before breaking would tear instantly during a Hulk transformation. Your standard cotton t-shirt is destroyed in milliseconds. Cotton boxers are unrecoverable.

Nylon

Tensile strength: approximately 75 to 94 MPa Elongation at break: 18 to 40 percent
Nylon stretches significantly better than cotton, which is why athletic wear often uses nylon blends. However, 40 percent elongation is still nowhere near sufficient to handle a 200 to 300 percent body size increase. Nylon underwear lasts perhaps a second or two longer than cotton before tearing.

Spandex (Elastane, Lycra)

Tensile strength: approximately 50 to 80 MPa Elongation at break: 500 to 700 percent

This is the answer. Spandex can stretch up to 7 times its original length before breaking. It is the only common commercial fabric that could realistically survive a Hulk transformation while maintaining structural integrity.
The honest physics answer to "why don't Hulk's undergarments tear off" is that Bruce Banner wears performance-grade Lycra or spandex underwear. The 2008 Incredible Hulk film even shows Banner shopping for special stretchy purple pants at a street market in Brazil, acknowledging that he has solved the wardrobe problem through deliberate fabric selection.
Marvel never explicitly states this in the MCU. But the materials science is unambiguous. A character with a transformation that increases body size by 200 to 300 percent would have to wear spandex or face genuine wardrobe failure every single time.

Bruce Banner is a brilliant scientist. Of course he optimized his underwear for transformation events. This is exactly the kind of preparation a genius with a Hulk problem would invest in.