A new born girl child was killed by a father. He told to the mother and police that, he that the baby was still born. Suspecting his statement the baby was taken for autopsy, where report was confirmed that it was a murder. The test performed by the medical expert was by taking a piece of lung and dropped in water where it:
Kept floating
The lungs of child starts working only after birth at that time the lung tissue are filled with air making it lighter and hence it floats on water. If the child was still born the lungs would have been heavier due to absence of air and would have settled down on the bottom. But it hasn't happened. So victim is father.