That is a very good question, and in fact the answer is very complicated!

First you might want to try an experiment:

Take a magnet (a refrigerator magnet will work fine) and try attaching it to all of the metal objects in your house (be careful you don't scratch your parents' car while you're doing this!)

What do you notice? You should find that in fact it doesn't attach to all of the metal objects, only to some of them. If you know a little bit about metals, you might notice that most of the metals that stick to the magnet are made of steel. Steel is what we call an ALLOY, a fancy word which just means that it contains a few different things mixed together, but it's mostly made up of iron.

So why does iron stick to a magnet? Well, first we have to think a little bit about how magnets work, and this is where it gets a bit complicated. We can think of a magnet as having a"north pole" at one end, and a "south pole" at the other end. (The names come from the fact that the Earth is in fact a magnet, with the "ends" of the earth's magnet at the north and south poles!) Whenever a magnet's North Pole comes across the south pole of another magnet it sticks to it. Whereas if a north pole comes across another north pole it pushes it away.

You can try this if you have 2 bar magnets; they stick to each other strongly in one direction, but not in the other.

Next you have to believe me that iron is actually made up of billions of tiny little magnets! Each one is just like the needle of a compass, but much smaller. (If you've heard about atoms, you can think of each atom as being a tiny magnet).

Now let's do a "thought experiment" (scientists use this a lot to try to figure out what might happen before they go into the lab and do a real experiment):

Let's imagine that we have a piece of iron, and we bring the north pole of a big magnet close to the top of it. (You might want to try sketching a picture of this as you're thinking through it; scientists do that a lot too!) Well, we know that the north pole of a magnet will push away any other north poles it comes close to. So each of the tiny magnets in the iron will turn itself round (just like a compass needle) so that its north pole is as far as possible from the north pole of the big magnet. Of course this means that the south poles of the tiny magnets lineup close to the north pole of the big magnet. And we know that south poles stick to north poles! So the iron sticks to the big magnet!