I specialise in ambigrams, and have been designing these since 2005. If the concept is new to you, an ambigram is the term used for two (different) words sharing the same ink.
The perhaps most typical example of an ambigram is that ‘transforms’ into another words when you turn it upside down. This is called a rotational ambigram. Other types of ambigrams include the mirror ambigram and the contrast ambigram.