this is probably allready planned but...
8 watermelon seeds = 1plantball
or 8watermelon slices, but, theres all that water to think about... 
and here's my original(?) idea...
8(or 9?) plantballs = 1slimeball, the idea behind it is that slimes are merely animated goop of ancient plant-life that had been buried in sediment, and then was rehydrated by ground water, and by random chance, came to life.* so, combining the plant matter, use Steve's super-awesome-crafting-skills** to convert said plantballs into a slimeball! HAYO! ***
cuz, like, most minecraft players haven't seen slimes, EVER!
*explains the special 'slime chunks' 
**magical?
***dont ask how steve simulates the whole 'millions of years of chemical changes' thing, or he shall be forced to terminate you. 