Where's the line between something being "stolen" and something being "inspired" by something else? People say that as soon as you "copy and paste" a line of c

Where's the line between something being "stolen" and something being "inspired" by something else? People say that as soon as you "copy and paste" a line of code that isn't yours, you're stealing. But it seems that without the "copying and pasting" that is inherent, not only in the learning process, but often in the process of innovation itself, we wouldn't have the products we have today. And what's the difference between copying and pasting something and creating something similar, but not an exact copy with similar code that you viewed and then re-typed in a similar way? Can one be outright theft and the other be acceptable?