The tear duct is what carries the eye's lubrication around the eye and causes any excess to come out at the inner corner. So if it's blocked, the lubrication and goop are just brimming up in her eye instead of coming out at the corner like normal.
The tear duct isn't responsible for producing tears, that's a common misconception. It basically just carries the moisture around. The eye has a tear "film" that produces the moisture, and if the duct is blocked, all that moisture just spills out over the rim.
CPST ***I can help keep your child safe in the car. Ask me a car seat question!***