const vs constexpr for variables in C++: what's the difference and which to choose?

Hello everyone! I'm learning C++11/14 and I'm confused about the difference between `const` and `constexpr` when declaring variables. I understand that both keywords make a variable immutable, but everywhere it's written that they are not the same thing. Here's an example that confuses me: ```cpp const int x = 42; // Option 1 constexpr int y = 42; // Option 2 const int z = foo(); // Works constexpr int w = foo(); // Compilation error if foo() is not constexpr ``` I've read that `constexpr` guarantees compile-time evaluation, while `const` does not. But then why is `const` needed if `constexpr` is stricter? And why do I see widespread use of `const` in real projects for variables that are clearly known at compile time? Another question: if I write `const int size = 10;` and