Lung float test
An old forensic test: lungs that have breathed air float in water, while lungs that never breathed are dense and sink.
The lung float test (hydrostatic test) checks whether a lung has ever been filled with air. Before birth, the lungs are collapsed and full of fluid, so they are denser than water and sink. Once a newborn breathes, tiny air sacs open and the lung becomes light enough to float.
It was described in the 1670s and first performed in 1681, and for centuries it was used in court to judge whether a baby had been born alive.
An everyday comparison is a dry sponge, which floats, and a sponge squeezed under water until it is full of water, which sinks lower: trapped air makes the difference.
The misconception is that the test is decisive. It is not: gas from decay can make a lung float even if it never breathed, and air can enter in other ways. A modern study still found about 2% false results.
