I also am not sure why it's intuitive at all. Article doesn't mention why - ruby has a parse phase, it doesn't scan your string byte for byte every time it runs looking for an interpolation. During the parse those "foo#{bar}" are replaced by something like 'foo'.dup << bar.to_str in the bytecode, but that happens only when parser hits #{ in a double quoted string, there's no penalty for that and it happens just once. Only really old, naive interpretators parse source code each time line is hit. (Or some weird ones, where interpolation can change semantics and argument count, like tcl and bash.)