Here's an admission - and I reckon most dentists do it too: I can't help but look at peoples teeth when I'm talking to them. And sometimes I can guess things very quickly about the person by what I see.

Take yellowy brown, glossy but dull teeth in a young person for starters.

Lets work through it:

Teeth are made mostly of dentine - a yellowish mix of calcium, phosphate and other stuff. Then there's a protective covering of crisp white enamel, the hardest material anywhere in your body. The thicker the enamel, the whiter and more vivacious the tooth. The thinner the enamel - the more the dentine shows through.

Remove the enamel and you get darker, duller looking teeth.

The commonest cause of enamel stripping - fizzy drinks.

When I see a younger person with shiny dark teeth (and usually a heck of a lot of fillings) - I'm usually right when I ask if they drink a lot of, say, coke.

So - if you want a bright, light smile - minimise your fizzy drinks.