Syringes work because of a thing called pressure. A syringe is basically a long plastic tube with a plastic plunger inside that sticks out from one end of the tube and can slide back and forth through it. The thin metal needle goes on the other end. It basically looks like this:
The plastic on the end of the plunger makes an airtight seal with the plastic tube, so when the plunger is pulled back, the inside of the tube is /totally/ empty. [not quite. see below/mw] We like to say that there's a vacuum inside. This doesn't mean that you could use it to clean your carpets... instead, it means that it's an empty space that doesn't even have air in it.
Well, there's liquid (like blood or medicine) on the outside of the needle, so it gets pulled into the empty space by the vacuum. This is because the liquid pushes up against the hole at the end of the needle, but since there's no air or anything there to push back on it, it rushes in.
When you press the plunger back down, it just pushes the liquid back out through the hole in the end of the needle.
(republished on 08/02/06)