Why do simple covalent molecules compounds have low melting and boiling points?

Define:A simple covalent compound consists of a small number of non-metal elements covalently bonded together. (1)For example, H2O , CO2, O2. (1)Relate to MP/BP: The small molecules have only weak intermolecular forces with one another. So the molecules can easily spread apart when kinetic energy is supplied by heating. This means little energy is required, and a liquid will quickly boil. (1)Draw energy increasing as heat increases.