This is just a thought that I had one day while watching The Hunger Games. If Foxface was so clever then why did she eat the berries. Not only that, but during the training days you actually see her matching up plants. I don't know, to me it just seems like she realized that she couldn't win so she just offed herself rather than waiting around for someone else to do it.
Clever:
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The way Foxface went out always stumped Katniss. She was so clever with everything that she did. And she was exemplary in plant recognition. So how did she die by night lock? The question buzzed around Katniss' head one night after another nightmare had woke her. Did she really not know that they were poisonous berries? The more she thought about it the more she started to wonder.
She had known what they were. She had to have known…then why? And then it hit her. Foxface knew she couldn't win, she was clever not strong. She knew she couldn't win.