Chef Henri must follow his recipe in the proper
order, or his mousse would not set properly every time he made
it. In math, if the order of operations is not used, people
may get different values for the same expression.
For example, if for the expression 3+5×4 you
multiply the 5 times the 4 first, you get 20, and then add the
3, the value of the expression is 23. On the other hand, if you
add the 3 and the 5 first, you get 8, and the value of 8 times
4 is 32. Therefore, we have a set of rules called the Order of Operations which help avoid confusion.