A 3-year-old child may have developed a habit since childhood. Is it always like this in infancy?

If you are teething, it may be itchy gums, and instinctively put your finger into your mouth to linger;

Or maybe you got something on your hand and smelled it.

The specific reasons need your observation. Children's habits are always closely related to life. Others can only provide possible reasons, and you can't draw a conclusion without observation.

Anyway, be sure to get rid of this habit! Eating bacteria is likely to make you sick. Even if you are not sick, pouting or grinning is not good. ...

I hope my answer is helpful to you.