Peni Peni is the yukkuri equivalent to the penis. It acts like (and superficially resembles) a real penis, but it injects a type of bean paste which is different from normal bean paste, because it can impregnate another yukkuri (from a different species, except for Yukkuri Rinnosuke. Attempts by a yukkuri to inseminate another of the same species inevitably fail without external "assistance") either by contact or by injection.
The Peni Peni is an extremely vulnerable organ, and as such it is normally stored inside the body in an inverted form (like a glove flipped inside-out); in this state it serves as Mamu Mamu. It is usually only deployed during copulation or when arousal is excessive, although Yukkuri Alice is often depicted with it deployed as its normal state.
Peni Peni only appears in some stories and drawings. Others using different methods of reproduction.
Since January 2009, several Japanese artists have drawn Yukkuri Alices as being able to spin their Peni Penis, either up and down as a flapping wing or in circles to make it work as a wheel, to move quicker on land, or as helicopter blades, to fly, or as an electric fan. They have also been shown using them as guns, allowing them to impregnate other yukkuri from a (sometimes considerable) distance.
An alternate reproduction method is the Rub Rub. Initially, the "Rub Rub" was the only way of Yukkuri reproduction, until Peni Peni became developed with the introduction of Yukkuri Alices.