This paper presents ten guiding principles for designing construction kits for kids: design for designers; low floors and wide walls; make powerful ideas salient - not forced; support many paths, many styles; make it as simple as possible - and maybe even simpler; choose black boxes carefully; a little bit of programming goes a long way; give people what they want - not what they ask for; invent things that you would want to use yourself; and iterate, iterate - then iterate again.