Related Clues:

• Jerk one's knee, perhaps
• Say ''ouch!'' when pinched, e.g.
• Punch back instinctively
• Do a double-take
• Write a letter to the editor, say
• Have a poor poker face
• Gasp, perhaps
• Change, chemically
• Say "wow!", e.g.
• Respond
• Make a face, say
• Flinch, e.g.
• Flinch, perhaps
• Show surprise, perhaps
• Smile, e.g.