heat capacity
Put a metal spoon and a mug of water in the same hot oven for a minute and the spoon turns scalding while the water barely warms. Water is stubborn about changing its temperature; the spoon gives in fast. Heat capacity is the measure of that stubbornness: how much energy you must pour in to raise something's temperature by one degree.
When you add heat to a solid, the energy goes into making its atoms vibrate harder — into more lattice vibration, more phonons. A material with a high heat capacity has many ways to soak up energy, so a lot of it goes in before the temperature climbs much. At everyday temperatures most solids share a similar simple value, but as they cool, quantum rules switch off the high-energy vibrations one by one and the heat capacity falls, dropping all the way to zero at absolute zero.
Heat capacity matters everywhere from cooking to climate to engine design, and historically its strange low-temperature behaviour was a crucial clue that energy comes in quantum packets. A common mix-up: heat capacity is not the same as temperature or as stored heat. It is the exchange rate between energy poured in and degrees gained — how much fuel buys you one degree.
A coastal town stays mild while an inland desert swings between baking days and freezing nights. The reason is the enormous heat capacity of the sea: it soaks up the Sun's energy by day and releases it slowly by night, barely changing temperature itself, gently steadying the air around it.
Water's high heat capacity is why the coast is mild and the desert is extreme.
Beware two close cousins: 'heat capacity' is for a whole object, while 'specific heat' is per gram or per atom. The huge thermal steadiness of the ocean comes from its sheer mass as much as from water's high specific heat.