Chapter 13, Writing Secure Contracts 

1. Msg.sender refers to the immediate caller while tx.origin refers to the original caller in chain. tx.origin is always an externally owned account whereas the msg.sender value can be a contract account or an externally owned account.
2. Recursion happens because the receive function in the hacker contract calls the withdraw function, which, in turn, calls the receive function unknowingly because it transfers Ether to the hacker contract.
3. Checks, effects, and the interaction pattern are three distinct stages in a sequence within a function that change the contract state and help transfer tokens and Ethers to other accounts securely. All incoming argument validation for correctness is executed as part of the check stage. This stage also includes validating the current state of the contract. By checking that the context and environment are at the conducive stage, the check stage ensures that nothing goes wrong from a state and incoming...