Colossus - the Rebuild

Colossus was a digital computer that contained 2400 valves and ran its programs as fast as a modern computer. It was designed in 1943 and made a significant contribution to ending World War 2. This is the story of that machine and the rebuilt version that you can visit at Bletchley Park (UK). How did Colossus came about, and how were valves made into computing elements?