Tickling is a spontaneous body reaction that makes the one being tickled laugh even if he or she does not want to. Since it is an uncontrollable reaction the one being tickled is often afraid of it if he cannot defend himself against it, as he loses control of his body. In some situations, the purpose of tickling is precisely to force the victim to open his or her mouth, in order to get him or her into trouble.
We have an example of this in the TV series “Floris” (1969) set in the 16th century.
Tickling in “Floris”
A girl and an old man are tied to poles: each of them holds a cable holding a precious map with mouth. Some evil individuals have captured them and want to destroy the map. Sadistically, they want the prisoners themselves to destroy it by dropping it into the fire.
To force the girl to open her mouth and destroy the map, the torturers begin in team to tickle her hips and under her armpits. The woman begins to laugh madly but tries desperately to keep her lips together.
One of the torturers, however, is an allied woman in disguise. She suggests that she give in to the tickling and open her mouth, that way a knight can retrieve her.