What causes random sneezing?

Why do some people sneeze when they don't have colds or allergies? My husband sneezes every morning, and I do whenever I look at the sun.

For something so common, we know very little about this. The nerve centre that, on stimulation, causes us to sneeze lies in the area of the brain at the top of the spinal cord, the medulla oblongata. The centre is part of the "parasympathetic" nervous system that helps to control the production of mucus in the nose and throat, the reaction of the stomach and gut to food, pupil dilation, heart rate, and blood flow to the sexual organs. In theory, when the parasympathetic nerves are in full flow, this can produce a sneeze. So bright light, drinking a whisky (one man's wife always knew he had sneaked a "dram" when she heard him sneeze) or even orgasm can provoke one.

I've started to feel pain on ovulating

In the last few months I've noticed what feels like period pain around the time of ovulation and it carries on until the end of my cycle. Is this related to age? I'm in my mid-40s.

Such pain in the middle of the cycle is usually due to the bruising on the surface of the ovary caused by the monthly separation of an ovum. However, it's unusual for it to start in your 40s - most women who have "mittelschmerz" have experienced it throughout their menstrual lives. So if it is new, something different has happened. I'm concerned, too, that the pain lasts for two weeks. Most mittelschmerz lasts for only a day or so. It sounds as if you need to have your ovaries examined. That means an abdominal and internal examination, probably followed by an ultrasound, to rule out an ovarian cyst or a patch of endometriosis. These symptoms are not directly related to age or hormonal changes, so do seek professional advice.

Is there a good book about depression?

Can you recommend anything to read about depression? My husband is very low after losing his job, and we both need advice on how to cope.

I haven't recommended a book here before, but I'm happy to do so now. Living With A Black Dog, by Matthew Johnstone (Constable & Robinson, £7.99), is a brilliant book of cartoons that tackles all the problems of couples such as yourselves. It's laugh...#8209;out-loud funny - which is pretty good for a book on depression - comprehensive and very helpful.

A weighty issue

We are always hearing that we get fat because we eat more than we should and exercise less. But aren't some people fat for other reasons?

The commonest cause is low thyroid gland function, but the link is still tenuous and there are usually other signs that make the diagnosis more obvious. Then there is Cushing's syndrome, in which the extra weight is laid down on the back and torso - it's due to overactive adrenal glands. Polycystic ovary syndrome produces obesity linked with over-production of androgens (male sex hormones). Drugs can put on weight, among them steroids, certain anti-psychotic drugs (such as clozapine and olanzapine), some anti-Aids drugs and some of the newer agents for type 2 diabetes. If you think one of these may have caused your weight gain, see your doctor, but don't hold out too much hope - they are all rare.

• Dr Tom Smith's latest book, Going Loco: Further Adventures Of A Scottish Country Doctor, is published by Short Books at £12.99. Do you have a question for Dr Smith? Email doctordoctor@theguardian.com