The ultimate, of course, is the UNARY computer. If
you look in the
"Feynman Lectures on Computation", Richard Feynman, 1996, you'll see
that he has you start designing a unary computer. Maybe not so easy
when all you have to work with is "1"! (It has to have a variable
word length, of course).
When all you have are ones (and nothing else to represent the absence of
a one), what do you use to delimit the start and end of variable-length
words?