Flush the toilet 47 million times and the whole of the shin-Ra water supply gets dirty!
Leave the toilet and go back to sector 6. Enter Don's mansion and on his bed you will find a "Horny potion". Later in the game when you get stuck at Goldsaucer with Aeris the command "Its Time" appears. Click this and you will recive "Baby" materia.

This materia has the ultimate power! The user shoots millions of babies at the opponent each of them cause around 8,000!

To power this up even more flush the toilet 37 billion times and the item "Jenova cells" appears! Use this on the Baby materia to shoot mutated babies!

This does 9999,9999,9999,9999,9999,9999,9999,9999,9999,9999,9999 each time!

Note: this works for males as well as females!

PS: I advise to look away from the screen if a male uses the baby materia....XD