Sperm banks store sperms in liquid nitrogen tanks, so that they "hibernate" at a low temperature of minus 196 degrees Celsius. Once reheated, the sperms can restore their vital functions. When donors donate sperms to the sperm bank, they need to sign a waiver agreement, that is, abandon all rights arising from the donation of sperms. Correspondingly, they are also exempt from various liabilities protected by law.