Covalent Bonding
When non-metals get together, they're too stubborn to give away electrons, so they share them instead - this creates covalent bonding. It's like two people sharing earphones because neither wants to give theirs up completely.
Simple molecules (like H₂O) have strong covalent bonds holding atoms together within each molecule, but weak forces between separate molecules. This means they have low melting and boiling points, don't conduct electricity, and are often gases or liquids at room temperature.
Giant covalent structures are completely different beasts. Imagine millions of atoms all holding hands in one massive network - that's what diamond and graphite are like. They have sky-high melting points because you'd need to break countless strong bonds to melt them.
Remember: Strong bonds within molecules, weak forces between molecules = low melting points
Graphite is special because it conducts electricity (unlike diamond) thanks to delocalised electrons that can move freely between its layers.