Get your toothbrush wet. Unscrew the toothpaste cap. Now squeeze.

If you’ve read the back of a toothpaste bottle, you know you’re supposed to squeeze out only a pea-sized amount per brushing session. (Littler kids should use a smear as small as a grain of rice!) But how many kids actually use that small amount? A study says “Not enough!”

The survey was done by workers at the Centers for Disease Control and Prevention. They surveyed the parents of more than 5,000 kids. They found that about 40 percent of kids between ages three and six use a brush full or a half-full of toothpaste. Experts say that makes it more likely those kids will have streaky or splotchy teeth when they get older.

Why the streaks? Most toothpaste contains fluoride. Fluoride is a mineral found in water and soil. More than 70 years ago, scientists discovered that people who drank water with more fluoride also had fewer cavities. So people worked to add fluoride to tap water, toothpaste, and mouthwash. Experts say fluoride helps keep teeth healthy. But too much will cause dental fluorosis—streaks and spots. Way too much fluoride can even create pits in teeth. But most cases of dental fluorosis are so mild only a dentist would notice.

“Fluoride is a wonderful benefit but it needs to be used carefully,” says Dr. Mary Hayes, a pediatric dentist in Chicago. Kids’ toothpaste tastes sweet. And little kids may want to use as much as they want. “You don't want them eating it like food,” Ms. Hayes says. “We want the parent to be in charge of the toothbrush and the toothpaste.”

Not everyone agrees about the benefits and dangers of fluoride. But most people can agree on this: Moderation is a good guide in life. Good things can become not-so-good if used too much.

It is not good to eat much honey, nor is it glorious to seek one's own glory. ― Proverbs 25:27