I live to move it move it!

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Across
  1. 3. "think fast!"
  2. 4. "Simon Says GO!"
  3. 6. FLEX UP!
Down
  1. 1. cortex: I live in your brain and tell your muscles what do
  2. 2. I keep your body upright and balance
  3. 5. I can be done through running, swimming etc.
  4. 6. motor: your need me to pick up a pencil