Snowballing is the human sexual practice in which one person takes another person's semen into his or her mouth and then "swaps" or passes it to the mouth of the other, or a third person, usually through kissing.
The term was originally used only by homosexuals. Researchers who surveyed over 1200 gay or bisexual men at New York LGBT community events in 2004 found that around 20% said they had one or more times engaged in snowballing.
In heterosexual couples, a woman who has performed fellatio may afterwards return the semen to her partner's mouth, mixed with saliva; the couple may then exchange the fluid several times, causing its volume to increase (hence "snowballing"). Many heterosexual men are uncomfortable with the practice.