Mercury, as you might expect given its speed, is the planet nearest to the sun and hence the most influenced by its gravity as it zooms around and about. At an average distance of 40 million km (25 million miles), even Venus, which is closer to the earth, doesn't reach Mercury's surface. Instead, it passes between us and the bright star at this distance every 48 hours or so. Its day is only 87 Earth days long, because almost all of that time is spent inside the shadow of the sun. Night comes quickly here due to how close they orbit each other. Mercury has no atmosphere or water vapor clouds like those found on Venus; instead, it has a cold crust of rock that reaches deep down into the planet's hot interior. It is thought that the intense heat from within makes lava flow on the surface, but not for very long since it eventually congeals due to the low temperature. The best place to see Mercury would be with a telescope, because even though it is the closest planet to the sun, it is always hidden behind the disk of sunlight that is our planet.