Related Clues:

• Smile, e.g.
• Block a punch, e.g.
• Acknowledge a stimulus
• Not just sit there
• Hit back, say
• Undergo chemical change
• Not keep a poker face
• Duck, say
• Gasp, say
• Fight or flee
• Flinch, for instance
• Sneeze, e.g.
• Wince or flinch, e.g.
• Flip, perhaps
• Wince, for instance