Citrus fruits are highly acidic. They can wear down the hard outer covering of your teeth, called enamel. That makes your teeth more vulnerable to decay. This doesn't mean you have to skip your morning OJ, though. Just be sure to brush afterward. And limit acidic drinks, such as coffee or lemonade, for the rest of the day.
If you have a sugary drink, drink it quickly instead of sipping it all afternoon -- and brush afterward, if you can.