"Sigma bonds occur when s orbitals, p orbitals pointing along the axis to the other bonding atom, or hybridized orbitals collide and overlap; the region of maximum electron density is between the two bonded atoms and lies along the axis between them. Single-order bonds are always σ bonds. Sigma bonds are like taking the index finger from each hand and pointing them directly at each other, so one fingertip touches another. Just as you can rotate your fingers without losing contact, bonded atoms can rotate freely about a sigma bond."