I'll tackle your question one by one, along with brief reasons along each answer. Then at the end, I'll go into a little bit more detail, as extra information. Sound good?
"What foods/drinks strengthen tooth enamel?"
--Fresh fruit, milk, butter, vegetable, carrots, beans, yogurt, cheese, whole grain cereals, fish, meat, etc. are all good for that purpose.
Just like all other parts in your body, your teeth depends on overall good nutrition to stay healthy.
Any food with fat-soluble vitamins (vitamins A, D, E, and K) will give you a fit and healthy body. What if you eat too much of them? That's the next thing to worry about, because most people DO NOT eat enough of these rather than eating too much. Also, don't forget calcium-rich food, too.
--Tap water has low acid, and it has fluoride, which helps to strengthen tooth enamel.
--Sugarless gum helps, too, as long you chew it, not swallow it (believe it or not, I still swallow gums sometimes because after a while, I totally forgot that I have gum in my mouth, so . . . yeah =P)
Sugarless gum has "xylitol." Xylitol is used to sweeten the gum and it has the effect of reducing the amount of bacteria in the mouth and it also helps buffer the teeth against the effect of acid.
Besides, chewing gum increases your flow of saliva, and your saliva can protect your teeth against bacteria.
"Which ones make it weaker?"
--junk food, soft drinks, soda, coke, potato chips, alcohol, food with a lot of salt, smoking, all those common-knowledge stuffs.
Just a warning, if you were to drink soft drinks, stay out of coke and pepsi; go with 7-UP, Sprite, basically those with the lighter color. Coke contains very, very high acid concentration. Coke can dissolve a nail within 2 to 4 days, just so you know =) Once in a while is ok, but if you drink them every day or even every week, uh, be careful there.
--Possessed fruits, dried fruits, canned fruits, etc. aren't replacement for fresh fruit. Remember the fat-soluble vitamins deal that I mentioned earlier? Only fresh food has decent amount of those vitamins.
-Bread, breakfast cereal, candy, and banana aren't so healthy either. They are within the category of "fermentable carbohydrates" food (definition below, in the more info section).
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More Information: What Causes Tooth Enamel to go Bad?
--Some foods breaks down once they go further down the digestive tract, but some begin to break down right when then enter your mouth. These kind of food are call fermentable carbohydrates.
Certain bacteria on your teeth produce acids using the sugars from those foods (by "sugar," I don't mean the regular sugar, but I mean the glucose, fructose, maltose and lactose). (And I forgot what those bacterias call so . . . sorry =( I can't tell you anything about them).
The acids then begin a process call "demineralization." It means that the acids begins to dissolve minerals inside the tooth enamel. If nothing is to be done, your tooth will begin to decay slowly.
YOUR GOAL HERE is to begin the process call "remineralization." It is the opposite of demineralization. You are trying to let your teeth regain the minerals that it lost due to the acid. Saliva, flouride, calcium, fat-soluble vitamins, and some food helps your teeth regain the lost minerals.
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