That's because the potato died.
It didn't just die, it went out in a messy, gross kind of way. It had sprouted a few little leaves, which then shriveled in on themselves and turned mouldy. The potato itself contracted and went wrinkly. I tried changing the water and moving the position of the potato around the house (high light, low light, dry air, moist air) but eventually just threw the experiment out. Oh, the feelings of failure when you can't complete a kids project!!
Anyway, today on the interwebs I stumbled upon this little gem that may have explained my loss. I'm wary of uncited articles, and in this day and age must always be skeptical of believing a 'real life' home video featuring a cute kid with a political message, however I'd prefer to go with the idea than against it. It's making me want to try my sweet potato project again, this time with locally farmed, organic and non-GMO produce, as well as plant out my roof top terrace with a well-stocked kitchen garden of my own. Have a look for yourself, and I'd love to hear your thoughts.