A Brief History of Time
All elementary particles (for example, the quarks) have something called spin. The spin of a particle shows us what a particle looks like from different directions. For example, a particle of spin 0 looks the same from every direction. A particle of spin 1 looks different in every direction, unless the particle is spun completely around (360 degrees). Hawking's example of a particle of spin 1 is an arrow. A particle of spin two needs to be turned around halfway (or 180 degrees) to look the same. The example given in the book is of a double-headed arrow. There are two groups of particles in the universe: particles with a spin of 1/2, and particles with a spin of 0, 1, or 2. All of these particles follow Pauli's exclusion principle. Pauli's exclusion principle says that particles cannot be in the same place or have the same speed. If Pauli's exclusion principle did not exist, then everything in the universe would look the same, like a roughly uniform and dense "soup".