When Spike is caught doing his thing with his... thing, Twilight forbids him doing it again until further research is done. With the help of a book, Twilight makes Spike conduct an experiment to discover the answers.

Author's note:
Twilight best matches her character from Witchcraft Is Magic, and may be easier to read with that in mind.
Spike is assumed to mature faster than ponies and is a fully consenting adult despite his young age. His status as a baby is from his conscious choice not to grow up by giving in to greed, as shown in Secret of My Excess.