Magnets are usually made of iron or a material that has lots of iron in it, such as steel. Although magnets are great at attracting most metal objects, you can hold a glass marble or plastic spoon against a magnet for as long as you like and nothing will happen. This is because magnets only attract other metallic objects containing iron, such as nails, jewelry and paperclips.

Much like the Earth, magnets have a pole at each end: a north pole and a south pole. Although you may not be able to tell the difference between a magnet's poles, they behave quite differently. If you put the pole of one magnet near the pole of another magnet, one of two things will happen. If the poles are opposites, they will attract and click together. If the poles are the same, they will push away from one another.

It doesn't matter whether you have a rectangular-shaped bar magnet or a curved horseshoe magnet, they both behave similarly. If you place an iron nail on a table with a magnet and slowly push the magnet toward the nail, eventually the nail will jump and stick to the magnet. You have just discovered the magnetic field.

Magnetic fields are invisible zones that surround magnets. Once a magnetic object enters the field, it is either attracted to or repelled away from the magnet.

If you have ever stuck an artistic masterpiece on the refrigerator, you already have some experience with magnetic fields. The paper gets held in place by the attraction between the refrigerator door and the magnet. As you may be able to guess, this means magnetic fields can actually pass through solid objects like paper.

Magnets can do so much more than just hang around on your refrigerator, though. Did you know they keep your refrigerator door closed, too? You may be surprised to discover how many uses there are for magnets.

When you leave your recyclables at the curb each week, you may not realize it, but they have a date with a magnet in their future! Recycling centers use magnets to help sort steel objects, such as tin cans, from other recyclables. The magnet won't help pick out the soda cans, however, because aluminum isn't magnetic.

Magnets can also be found inside computers, doorbells and soda machines. Magnets help electric can openers hold cans in place and compasses point north. If you look closely, you'll discover magnets tucked inside small pockets at the bottom of most shower curtains. The magnetic attraction keeps the shower curtain inside the tub, so you don't end up with a flooded floor.

When it comes to some of the strongest magnets around, you won't find them in a recycling center or hanging on someone's refrigerator. You'll find them in a hospital.

An MRI (magnetic resonance imaging) machine uses powerful magnets and radio waves to give doctors a glimpse inside the human body. The magnetic strength of an MRI's magnetic field is 20,000 times stronger than the Earth's magnetic field. As you can imagine, that's one serious magnet!

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Put on your hiking boots and join us in Wonderopolis tomorrow, as we trek across China to check out the greatest wall of them all.