Constant folding vs. Constant propogation

What is the difference between constant folding and constant propogation? They both seem to do the same thing, instead of saving constants into stack or evaluating a full arithmetic expression, they simply replace it with the result which can be obtained at compile time. What is the difference between two?

Based on these two websites: http://www.compileroptimizations.com/category/constant_folding.htm and
http://www.compileroptimizations.com/category/constant_propagation.htm

The difference is that constant propogation is not saving a variable to the stack, because we know its a constant (like x = 10;) and can simply plug it in everywhere (say if you had an expression y = x + x + x => y = 10 + 10 + 10) it is used in machine code. Whereas, constant folding is simply evaluating expressions that use constants and substituting the result into the machine code (like y = 10 + 10 + 10; => y = 30;)

StackExchange Computer Science Q#79824, answer score: 9