Sarah Jacobs, of Kirkwood Intermediate School, asks :-

Why are people sick when they put their fingers down their throats?

Mike Ardagh, a specialist in Emergency Medicine at Christchurch Hospital, responded.

When something touches the back of your throat firmly enough, or for long enough, a course of events is set into action known as the vomiting or gag reflex.

You may know of the knee jerk reflex, when a doctor taps someone's knee their leg moves automatically. This reflex occurs because the nerves of the knee feel the tap, and then tell the nervous system which reacts by making the muscles move the leg. This is a simple reflex. The vomiting reflex is a little bit more complicated. When the sensitive lining of the back of the throat is touched, the nerves in that lining then tell the nervous system and the nervous system decides that a vomit might be in order. It then instructs the muscles about the stomach and abdomen to empty the stomach of its contents.

This reflex probably evolved to protect against choking on things stuck in the throat by clearing them away with a vomit.

The gag reflex you asked about is reasonably simple but other vomiting reflexes can sometimes be quite complicated. Some people vomit at certain sights and smells and others vomit with emotions like fright or sensations like pain. Just to complicate things further the nervous system will occasionally throw in a dizzy spell or even a faint to accompany the vomiting reflex in some susceptible people.