Why do girls stick out tongue?

for years i have noticed some girls stick out there tongue at guys...
At first i thought it was because they liked them, but i am not sure anymore.
Update: well reasontly there was a girl that i was on a road trip with that every time i looked at her, she stuck out her tongue. but she was normally in a diffrent car than me. So it was not like i could see what she was thinking.
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