Share and Share Alike

Me: walking out of the girls' room at bedtime.
Gwen: "Mom, I need a drink of water."
Me: "K, just a sec." (I remember doing this to my mother nightly... mental note to thank her next time I see her...)
Moments later...
Me: Handing Gwen a water bottle, which I (stupid mother) know is Tula's, but don't care, because it's bedtime and I am therefor, almost off duty...
Tula: Shrieking from the top bunk: "NOOOOOOO!! That's MIIIIIIINE!!"
Me: "Tula, really! Can you please just share?"
Tula: "NOOOOOOOO! I HAAATE Sharing!! I don't EVER want to share!" Sobbing...
Gwen: Finishing her drink of Tula's water, satisfied and smirking.  Knowing full well that I had, this time, been the one to make Tula absolutely belligerent.
Me: Handing Tula her water bottle.
Tula: Taking a drink... Pause... WAILING... "It tastes like my sister's BREATH!!!" Angry sobbing...
Me: Dumping out water bottle.  Refilling water bottle.  Returning to belligerent child.  "Is that better?"
Tula: Blotchy, tearstained... "Yes." Sniff.