British physicist Lord Rayleigh is best known for his discovery of argon and for explaining, in 1871, why the sky is blue. But he also puzzled over this: Rayleigh knew that a kettle makes that sound when steam jets through the hole in a thick lid that has a gap in the middle. He speculated that the jet becomes unstable inside that gap, setting up an acoustic feedback loop within the gap. But he couldn't prove it. Now two engineers at Cambridge University claim to have solved the puzzle—and proved Rayleigh wrong. The work is in the journal Physics of Fluids. The engineers found that a kettle actually whistles in two distinct ways. It starts off with air vibrating in the gap between the layers of the lid, like when you do this and this [sound of whistling]. But as the pressure builds, vortices of steam peel off from the jet exiting the lid. Each vortex creates sound waves at a frequency that depends on the length of the spout and the pressure inside it. Rising temperature means rising pressure, which produces a rising whistle. Which means it's time for tea.