Why does my 2 1/5 mth old suck on her fist?

This is my daughter's favorite past time still at nearly 5 months. She has teeth now though and bites herself. It is part of your baby discovering her hands! She enjoys the feeling of her hands and realizing that she is making them feel that way and it also pacifies her sucking reflex. Wait till she starts teething and chewing on them, you'll have a drool monster explode in your house!