Mercury is probably a big metal ball covered by a rocky crust. The core (which is the big metal ball) is made of nickel and iron - just like the Earth. Scientists think that part of this core is liquid, since Mercury has a fairly strong magnetic field. (Remember that stars and planets can be big magnets.) Scientists thought it might not have a magnetic field at all!

No, you can't live on Mercury. First of all, there's no water and no air to breathe - not even poisonous air.  Mercury is missing something called an atmosphere. You can think of an atmosphere as a protective shield of stuff (like gasses) that surrounds a planet. The reason that Mercury doesn't have an atmosphere is that it's just not massive enough - not big enough to have a gravity strong enough to hold gasses close to it. (Just check out how much you would weigh there to get an idea about the gravity thing.)

Also, it's way too hot in the daytime and, surprisingly, way too cold at night! How can it be cold at night when it's so close to the Sun? One of the jobs of an atmosphere is to hold in the warmth of the daytime. Since Mercury doesn't have an atmosphere, it is exposed to the cold of space every night... and the nights are really long! It's really cold out in space!

By the way, Mercury is NOT the hottest planet - even though it's the closest to the Sun! Venus totally  beats it. Why? Go see for yourself!

To escape Mercury's gravity and get out into space, a rocket would have to travel at a speed of 9,600 mph or 4.3 km/sec. That's only about 148 times faster than your parents are allowed to drive on a U.S. highway! We could easily come home from Mercury if we ever decided to visit.

Mercury is full of big craters! This is where things from outer space, like meteors, have crashed into it. Stuff like this has crashed into all the planets just as much... but we don't see as many of the craters as on Mercury. This is because things like water, wind and general stuff that goes along with having an atmosphere smoothes the craters out after awhile. So, with no atmosphere, we get to see all the scars of everything that has ever crashed into Mercury and that's pretty cool.

Did you notice that Mercury doesn't have any moons? This is because it is too close to the Sun. The Sun is so huge (massive) that its strong gravity would suck a moon away in a second!

Sun | Mercury | Venus | Earth | Our Moon | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto

Sources:
In Quest of the Universe, 2nd ed. by Karl F. Kuhn
Voyages Through the Universe, 2nd ed. by Fraknoi, Morrison and Wolff
Universe by Kaufmann
The Complete Idiot's Guide to Astronomy by Christopher De Pree and Alan Axelrod
The Astronomy Cafe by Sten Odenwald