Debatri Chattopadhyay

Waves sort pebbles on a sandy beach. Small stones lie close together, nearly identical in size. Farther from shore, there's a scatter of larger and more varied ones. And right at the waterline, where the surf is strongest, the biggest and most rounded fragments pile up. Nature sorts black holes by mass in much the same way when they merge in pairs.

Think of a merry-go-round: it's easier to spin a light one, harder to spin a heavy one. Neutron stars, the ultra-dense remnants of exploded stars, behave in a similar way. Astronomers looked at fifty binary systems where one neutron star orbits another and noticed: the heavier the star, the longer its spin period. The link isn't strict, but it's statistically noticeable.