It's not a puzzle. it is the SHA256 hashing algorithm designed by the NSA.
You keep hashing the block data plus a nonce until you find a value lower than the target. That's the solution.
Oh the target is continually adjusted according to the network hashing rate- every 2016 blocks.
You haven't read the right material yet.
To be exactly this is a Puzzle.. its called cryptographic Puzzle.. this is used as "Anti dos" atack on Webservers for example.. the Client has to "Invest" computimg Power to be able to communicate with Web Server..
It Works on the right principe.. you have to find Hash with X starting 0's... X equals difficulty.
It's not a puzzle. There is no amusement or entertainment in brute-forcing a mathematical function. This was a term coined to make it easy to explain to the layman. It's as much a puzzle as is grinding coffee to make espresso.
Edit: IMHO
Yes, finding the next block is not a true puzzle, but is basically just randomly guessing.
Data from the previous block is hashed with a random element (the "nonce") in order
to get a result that the whole network can verify publicly.
A puzzle would imply that the solver is attempting to "find a block" by reasoning.
It is really closer to a trial and error race of guessing. Guess right, you win.
A cryptographic puzzle is more inline with the encryption that which Bitcoin's
public/private key security relies. When it comes to mining, it is just simple hashing.