Why does fat sizzle loudly when it hits a hot pan?

When fat or any moisture-containing food hits a hot pan, the water content inside instantly vaporizes into steam. Fat from meat or other sources always contains some water, and when that water turns to steam rapidly, it expands with force, creating the loud sizzling and popping sounds you hear. The noise is essentially countless tiny steam bubbles bursting at the surface. Pure fat with no water actually sizzles much more quietly. The loudness increases when there is more water present, which is why wet or thawed meat sizzles more aggressively than dry meat. The high temperature of the pan, typically well above water's boiling point of 100 degrees Celsius, causes this near-instantaneous vaporization that produces the characteristic sound.