What makes Bitcoin transactions secure and trustworthy? Computer security and cryptography expert Zulfikar Ramzan explains the workings of the transaction blockchain in this video, one of a series that are featured on the online learning site Khan Academy.

Understanding the transaction blockchain, Ramzan says, helps to illustrate "how somebody might try to game or defraud the system and why that's not only mathematically hard to do but why there's actually an incentive -- actually an economic incentive in the Bitcoin system -- for different people to behave honestly."

That incentive comes through the process of mining, which involves solving a so-called "proof-of-work" puzzle. Whoever solves the puzzle first "mines" a new batch of bitcoins. While a single miner trying to solve a proof-of-work puzzle could spend a year or two looking for the answer, the network of miners is so large that someone, somewhere finds a solution, on average, every 10 minutes. Over time, the chain of puzzles and solutions becomes so long -- and so many people have contributed their work and computing power to it -- that it becomes increasingly unlikely to be tampered with.

"The more work that went into the overall chain, the more trust they'll have in that transaction," Ramzan notes. "There are no known shortcuts for solving these puzzles ... To succeed in a proof-of-work is kind of like winning the lottery."

Watch the whole video here: