Chapter 7, Functions, Modifiers, and Fallbacks 

1. There are four types of scope visibility modifiers available in Solidity – private, internal, public, and external. Also, state variables cannot have external scope visibility. 
2. The fallback and receive functions are special Solidity functions that are executed automatically, based on certain conditions. A fallback function is executed if a function call on a contract is made and such a function does not exist, whereas a receive function is executed when any value in terms of Ether, wei, or gwei is sent to a contract without any function invocation – for example, using the send or transfer functions.