The 350-year-old mystery of why pendulum clocks hanging from the same wall synchronize over time may finally be solved, scientists say.

In 1665, Dutch physicist Christiaan Huygens, inventor of the pendulum clock, was lying in bed with a minor illness and watching two of his clocks hanging on a wall, said Henrique Oliveira, a mathematician at the University of Lisbon and co-author of a new study detailing the findings. Huygens noticed something odd: No matter how the pendulums on these clocks began, within about a half-hour, they ended up swinging in exactly the opposite direction from each other.

Researchers have calculated that, as pendulums move back and forth, sound pulses could travel through the wall from clock to clock. These pulses can interfere with the swings of the pendulums, eventually causing them to synchronize.