Why do some people slip while the majority do not?
Slissing is a speech impediment that is common in small children who learn to talk.
A child will start to piss when the air from the vocal cords goes out through the wrong places, such as between the side of the tongue and the molars, causing whistling hisses.
Some people do it on purpose
Swallowing can have a physiological cause, such as a too short tongue belt, which makes it difficult for someone to hiss, or have to do with the position of the teeth.
It can also be learned. People from Andalusia in Spain are deliberately slitting because their language uses the slis-s.
In children, swallowing usually ends around the age of five. If not, a speech therapist can offer a solution.