Possible Clues:

• Takes the bait, say
• Flinches or blinks, say
• Loses the head as he is taken away from teachers
• Undergoes a chemical change
• Jumps back, say
• Flinches, for instance
• Responds to trace elements switched with sulphur added
• Hits back, say
• Screams or jumps
• Shows shock, e.g.