Three measures, three denominators, and the denominator is the whole story.

Molarity: $C = \dfrac{n}{V}$, C equals n over V. $C$ is the molar concentration in mol/L, $n$ is the amount of solute in moles, and $V$ is the volume of finished solution in litres — not the volume of solvent you started with. It is the working chemist's default because volume is what a pipette measures.

Molality: $b = \dfrac{n}{m_{\text{solvent}}}$, b equals n over m-solvent. $b$ is in mol/kg, $n$ is again the moles of solute, and $m_{\text{solvent}}$ is the mass of solvent alone, in kilograms, weighed before the solute ever arrived. Masses do not expand when the room warms, so molality holds still where molarity drifts — which is exactly why the colligative laws in the next lesson are written on it.

Mass percent: $c = \dfrac{m_{\text{solute}}}{m_{\text{solution}}} \times 100\%$. Here $m_{\text{solute}}$ is the solute's mass and $m_{\text{solution}}$ is the mass of everything in the beaker, solute included. The classic slip is dividing by the solvent instead — the solute is part of what it is a percent OF, so it belongs downstairs too. Notice the family resemblance: molarity's denominator is the solution, molality's is the solvent, and mass percent goes back to the solution. Read the denominator, name the measure.