The mechanism of Big Ben is so precise that adding or removing a single penny coin from the pendulum changes the clock's speed by 0.4 seconds per day. A small stack of pennies sits on the pendulum of Big Ben, and its mechanism is finely tuned enough that even the minor weight change of a coin affects the clock's timekeeping.

5. https://www.youtube.com/watch?v=wjYnddeEfao
https://www.youtube.com/watch?v=XgbTh1BCciA

16. There is a small stack of penny coins on the
pendulum of Big Ben. The mechanism is so precise
that adding or removing a single coin changes the
speed of the clock by 0.4 seconds/day.