The solar system appears to have formed from a disk of dust and gas drawn together by gravity. About 4.6 billion years ago a cloud of gas and dust collapsed under its own gravity. As it shrank it spun faster and flattened into a disk, with the Sun forming at the hot center. Grains in the disk clumped into larger and larger bodies that became the planets. Because they formed from one spinning disk, the planets all orbit in the same direction and in nearly the same plane. Asteroids and comets are leftovers from that time. This week the crew models the disk with a stirred bowl of water.