In 1939, child psychologist, author and pioneer toymaker Hilary Fisher Page developed a visionary new toy: hollow, plastic building blocks with four or eight studs that enabled children to build stuff with them, from little toy houses to huge plastic skyscrapers. Page himself built these structures for Kiddicraft's stand at the 1947 Earls Court Toy Fair. You know those impressive LEGO displays of entire cities they have in their parks and stores? Kiddicraft was doing that before LEGO bricks even existed. Here's a picture of some Kiddicraft buildings:
You could also build extremely realistic-looking toddlers.
In the same year, LEGO's founder Ole Kirk Christiansen bought a plastic injection machine from a salesman who demonstrated the type of toys that could be produced with it by showing him some Kiddicraft play sets. This was only meant to be an example of the machine's capabilities, but Christiansen had a better idea: doing the exact same thing and calling it by another name.
LEGO went into full Thomas Edison mode, copying the Kiddicraft play sets right down to the little doors and windows that came with them:
Kiddicraft on the left, LEGO on the right.
The rest, of course, is history. LEGO went on to become a household name to such an extent that it is now impossible to go to the bathroom in the middle of the night without stepping on at least one of their products on the way (even if there are no children in your house). Page, in turn, ended up committing suicide in 1957 as his company went down the toilet. LEGO respected his death by gleefully buying the rights to the rest of his ideas in the '80s, then doing nothing with them.