Installing Windows 10 for free?

How can I do to install Windows 10 for free in my laptop? I mean, without having to enter a serial number. I thought it was easy but now I see it rather difficult. I saw a youtube video where it says that you install it and the installer asks for a key (aka serial number). But you don't give it. That the only thing that happens when you use the system without the key is that certain things you cannot do them. But that's all. They are not important things.

Well, forget this. My question stands.