This is very common and is normally caused by a blocked tear duct. This often gets better on its own or may need syringing at a hospital eye department. Sometimes a more complicated operation called a dacrocystorhinecstomy (DCR) which creates a new tear duct is required.
Watery eyes can also be caused by infection, irritation(by a foreign body), or inflammation. Ectropian may also cause watery eyes- see information on ectropian.
Often children have this condition and it is mistaken for an infection. In fact the tear ducts are not often fully open until 12 months of age, they therefore easily block. If the child's eyelids are cleaned with cotton wool and sterile saline, this often helps and it also reduces risk of secondary infection. This should also be done if you notice crusty lids and yellow discharge. Sterile saline can be obtained from a chemist or opticians in aerosol form and is used normally for rinsing soft contact lenses. Oxycept saline by AMO is one example. See information on blepharitis.
All information is for reference purposes only; if you have any concerns we recommend that you visit a qualified optician.