The electrons in an atom can only have certain energies. When an electron drops from a higher energy level to a lower one, the atom gives out a photon with exactly the energy difference, so only certain wavelengths appear. These lines are a fingerprint of the element. A mixture shows the lines of both elements.

Hydrogen and helium follow real measured lines. For this game, the other lines are simplified, and a few lines from different elements fall very close together (within about 3 nm). No two answers show the same set of lines, so the puzzle can always be solved if you count the lines carefully.

Calcium and magnesium are metals, so in a real discharge lamp they would be vapours.