I washed some apples and served them on a hand-carved soapstone platter (see photo). When we began to eat them, we noticed the apples had left blue stains on the plate. What could have caused this?

Tony Wills, London, UK

When silver nitrate comes into contact with skin, the skin acquires a brown/black stain that is very difficult to remove. What compound is the brown/black substance, and why does it form in the skin? Also, what makes silver nitrate corrosive?

I drink my tea strong with very little milk. I use the same mug all the time and I don't allow my wife to bleach it. The inside builds up a satisfying coating which resists washing up liquid and scouring.

My darling beloved's own tea is what I call children's tea - weak and milky. Her cup never gets stained.

What is the brown stuff ('tannins' will not count as an answer)? Does milk somehow neutralise it? How does it get out of the teabag if it's so insoluble? Does bleach remove it or just remove the colour? Is there a good way to get rid of it?