Both my kids had almost completely black teeth around age 2. It was definitely not caries. All the dentist could tell us was that it is harmless and will eventually disappear (which it did) - but which organism (bacterium?) causes these stains, and why do only some children get them?

Here in Canada, many children grow up in bilingual households. How do their developing brains learn to understand that while 'Hello' and 'Bonjour' mean the same thing, they can only be used within another set of words? Is this a problem? Are there 4 year-olds speaking with a mix of 2 different vocabularies?

The mark test is a test where a mark otherwise undetectable by touch or smell etc. is placed on the face (or another part of the body invisible without the aid of a mirror).  If the subject, after viewing themselves in the mirror, moves their hand (or another part of them if its an animal) up towards the mark on their own body, they are said to have self-recognition.  Children usually start being able to pass the test (but not always and not perfectly) after about 18 months.

When I was a kid, I remember thinking that adults were rather boring for not enjoying the feeling of dizziness like I did, and I vowed to always enjoy it. Now, as an adult, I can't stand it - it makes me want to throw up. It seems many other adults feel the same way. Why is this? Does something change in us as we age?

Ivan V., Mexico City, Mexico