Its a childhood classic, “guess what number I’m thinking of”. Its funny to think, but Bitcoin is basically a computerised version of that game.
In the childhood game, you may play with a friend and state that the number is between 1 and 10: in this case each round of the game won’t last long. How could you make the game longer? Easy, by increasing the difficulty to make them guess a number between 1 and 500.
Let’s say ideally you want a game to last for 10 minutes, and you have 5 players. If each player can make 60 guesses per minute, in 10 minutes each person will make 600 guesses, that’s 3000 between the group. Double this number and you know that for the game to last around 10 minutes, your target difficulty is a number between 1 and 6000.
https://seebitcoin.com/2016/06/understanding-bitcoin-the-childhood-game-that-rules-the-network/