Chlorine and Tooth Discoloration

I took my 8 year old to the dentist today for a check-up. One of our concerns was the brown discoloration on her two front teeth. The first question the dentist asked was "Is she a tea drinker?" (No). The second question stunned me, "Is she a swimmer?" Apparently a few of his swim team patients over the years have experienced discoloration. It's not permanent (he was able to scrap her's off). He didn't explain the chemical reaction taking place. I have never heard of this before, so I thought I'd put it up here. Have any of you had this experience either with yourselves or your kids? And can anyone explain what the chlorine/dental connection is or could be? :coffee: