The age of the earth
(Thanks to Yeshu Kolodny) We now know that the age of the earth is 4.54±1% Billion years.
From Wikipedea: In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that fixed the age of Earth at between 20 million and 400 million years. He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface to cool to its present temperature. His calculations did not account for heat produced via radioactive decay (a process then unknown to science) or convection inside the Earth, which allows more heat to escape from the interior to warm rocks near the surface.
Test your intuition/knowledge
What was the main reason for Lord Kelvin’s wrong estimation
a) Radioactivity – Heat produced by radioactive decay; this was a process unknown to science for decades to come.
b) Convection – The transfer of heat not through radiation or heat-conduction but through the movement of hot parts to the surface; this is a process familiar in home cooking.
Here is a link to a nice physics “test your intuition” over The Quantum Pontiff
Combinatorics and more 