Because it is almost impossible to guess the input of a hash calculation, there is no straightforward way for a computer to calculate these additional numbers (which is called nonce - a number used only once).

The only way to solve this puzzle is to try different nonce values. As a result, this inevitably takes time and energy to solve and, if more computers try to solve the same puzzle, they can guess the right nonce faster. Because a hash can be calculated instantaneously, once the nonce values are found, it is very easy for other computers to confirm that the puzzle is solved.

When the puzzle for a new block is solved, that block can be added to the blockchain.