A single bond lets the groups at its two ends spin. A double bond holds them in place, and that one fact gives some molecules two different forms.
In ethane, C₂H₆, the two carbon atoms share one pair of electrons. That pair sits right on the line between the two atoms, so each CH₃ group can spin around the bond like a wheel on an axle, and the pair stays shared all the way round. At room temperature, the two ends of an ethane molecule turn against each other billions of times a second. That is why a chain of single bonds can twist and bend in countless ways and still be the same molecule.
In ethene, C₂H₄, the carbons share two pairs. The first pair sits on the line between them, as in ethane. The second pair spreads out into two clouds, one above the line and one below it. All six atoms of ethene lie flat in one plane, as lesson 3 showed, and the clouds stand above and below that plane.
The second pair holds the two carbons together only while the clouds from both carbons overlap, and they overlap when both ends of the molecule lie flat in the same plane. Twist one end a quarter turn and the clouds pull apart, which breaks the second bond. That takes a lot of energy, so at room temperature the ends of a double bond stay locked.