The `SENTINEL` is a [constant][1] that has the specific purpose of being the value that breaks out of a loop. You define it at the top of a program next to the other constants in all caps like this: var SENTINEL = 0; The value of sentinel will usually be `0` or `-1`. ---------- Example of a program that uses a `SENTINEL`. This program asks the user for numbers until they enter the `SENTINEL`. Then, it prints the sum of all the numbers they entered. var SENTINEL = 0; function start() { var sum = 0; while (true) { var num = readInt("Add a number (" + SENTINEL + " to stop)."); if (num == SENTINEL) { break; } sum += num; } println("Sum: " + sum); } Note how it uses an if statement that breaks out of the loop when you enter the `SENTINEL`. You *could* just use a number instead of the `SENTINEL`, but that would be a [magic number][2], which is not as good style. [1]: http://codehs.com/glossary/term/13 [2]: http://codehs.com/glossary/term/14
