Related Clues:

• Traces changes, as responds to stimulus
• Punches back, say
• Does a spit take, say
• Flinches, say
• Acts in response to something
• Gasps or grimaces, say
• Laughs or cries, maybe
• Does a double take, say
• Fails to stay poker-faced
• Loses the head as he is taken away from teachers
• Doesn't keep a straight face
• Undergoes oxidation, say
• Jerks, say
• Changes, chemically
• Gives feedback