Same July sun, same beach: the sand scorches your feet while the water stays cool. Both collected heat all morning — but water is thermally stubborn: it takes a small mountain of energy to move its thermometer one degree. That stubbornness has a name, the specific heat capacity $c$, and it is the star of this chapter.

Keep the two ideas apart. Heat $Q$ is energy on the move, counted in joules — it flows. Temperature is a thermometer reading, in °C — it describes. The formula that connects them is $Q = mc\,\Delta T$ (read aloud: Q equals m c delta-T): the mass $m$ being warmed, the price tag $c$ per kilogram per degree, and $\Delta T$, the change in temperature — final minus initial, the delta you met in kinematics. For water, $c = 4200\ \mathrm{J/(kg\cdot{}^{\circ}C)}$ — among the highest of any common substance, which is why the lake outlasts the sand.