While you can use values like the string "Hello World" from the previous
chapter or a number like 3.14 directly in code, usually you assign values like
these to either a variable or a constant. You can give values a convenient
name using variables and constants in Swift.
Variables can change their values over time, while constants get an assigned
value and keep that value throughout the execution of a program. Both
variables and constants can store only one type of data.