In order to get a useful amount of gaseous fuel into a reasonably-sized tank, you have to liquify it. Some fuels are easier to liquify than others. According to the textbook Organic Chemistry by Joseph M. Hornback, propane has a boiling point of -44° F (-42° C) at atmospheric pressure, but methane (natural gas), has a boiling point of -260° F (-162° C) at atmospheric pressure. This means that methane has to be cooled to a much lower temperature than propane in order to be turned to a liquid that can be stored in a tank. Propane molecules consist of three carbon atoms bonded in a chain with eight hydrogen atoms bonded to these carbon atoms. In contrast, a methane molecule is just one carbon atom bonded to four hydrogen atoms. Methane molecules have a high degree of symmetry. As a result, they do not have a permanent electric dipole. Bonding between permanent dipoles is the dominant bonding mechanism between molecules as they liquify for many substances such as water. Methane's symmetry, and therefore lack of a permanent electric dipole, means that its molecules can only bond through a much weaker effect known as the London dispersion force or the van der Waals force. In this effect, molecules induce temporary dipoles in each other, and these dipoles then bond. Because this bonding mechanism is so weak, the methane molecules have to be cooled to a low temperature until they are still enough to bond and form a liquid. In contrast, propane does not require as low a temperature to liquify.