Why teeth become stained.

Two teeth can be cleaned identically, fed identically, and end up looking nothing alike. The reason sits in three places: the film that coats them, the colour inside them, and the way time edits enamel from the outside in.

A film called pellicle ¶

Within minutes of brushing, a thin protein layer re-forms on the surface of every tooth. It's called the dental pellicle — about half a micron thick, made mostly of salivary proteins. It's a feature, not a bug. The pellicle is what protects enamel from being eaten away by acidic foods.

It also catches everything you eat and drink. Tannins from tea and coffee. Chromogens from beets, berries, curry, tomato. Phenols from red wine. Tobacco residue. These molecules bind to the pellicle within hours and oxidize over days, turning faintly darker as they sit. The film keeps re-forming, and the residue layers.

Polishing brushing removes the day's residue. It does not always remove yesterday's. That accumulation is what you see as the slow yellow drift over a season.

Extrinsic stains ¶

Anything on the pellicle is extrinsic — on the outside. It's the easier kind of stain to address. Mechanical abrasion (a soft brush, a low-RDA polishing paste) clears most of it. Hydroxyapatite and certain enzymes lift the rest. Whitening toothpastes live almost entirely in this category.

If you brush twice a day, floss, and don't drink coffee, you're unlikely to see meaningful extrinsic stain unless something else is going on. If you drink three cups a day or smoke, you'll see a steady extrinsic build that no amount of polishing fully erases between meals.

Intrinsic stains ¶

Intrinsic discolouration lives inside the tooth — in the dentin or the enamel matrix itself. It builds over years. Common causes:

• Cumulative dietary chromogens — the long, slow version of extrinsic, where pigment diffuses past the pellicle into porous enamel and stays.
• Medications — tetracycline taken in childhood (when teeth are still developing) leaves grey-brown bands. Iron supplements can leave dark deposits.
• Fluorosis — too much fluoride during childhood tooth development leaves chalky white or brown mottling.
• Trauma — a knock to a front tooth can damage the pulp, causing the dentin to darken from inside.
• Ageing — the most universal one. More below.

Why ageing teeth read yellow ¶

Enamel is translucent. Underneath it sits dentin, which is naturally yellower and slowly accumulates colour from food, drink, and the body's own pigments. As enamel ages — thinned by years of chewing, micro-abrasion, and acid wear — more of that dentin colour shows through. The tooth doesn't go yellow. The cover comes off.

A tooth at sixty is the same tooth it was at sixteen, with less curtain.

This is also why peroxide whitening works at any age. The pigment in the dentin is the actual target. Lift the dentin a few shades and the visible result is striking, because there's less enamel mediating it.

Why some smiles stain faster ¶

Three variables you don't choose:

• Enamel thickness. Some people inherit thicker enamel. More translucency mediation. Less dentin showing.
• Enamel porosity. Higher microporosity (post-orthodontics, mild fluorosis) means more channels for pigment to enter and lodge.
• Saliva composition. Salivary flow rate and protein profile change how thick the pellicle gets and how much it catches.

You can match coffee intake with someone and end up with very different teeth a year later. None of this is a moral failing. It's biology.

What you can do about it ¶

Three things make most of the difference:

1. Don't brush in the wrong window. Acidic foods (citrus, wine, sparkling water) temporarily soften enamel. Brushing immediately mechanically removes the softened layer. Wait 30 minutes.
2. Polish, don't sandblast. High-RDA pastes used daily thin enamel. Use them 2–3 times a week, not every brush.
3. Address extrinsic and intrinsic separately. Extrinsic: daily care. Intrinsic: a periodic whitening course — peroxide does the work brushing can't.

The film is the canvas. The colour you can't reach is underneath.

Next, the rituals piece: the two-minute oral care ritual.