Related Clues:

• Respond to a stimulus
• Show surprise
• Respond to stimuli
• Jump at a noise, e.g.
• Show one's emotions
• Strike back, say
• Retaliate
• Act in opposition
• Strike back, e.g.
• Hit back
• Do a double-take, e.g.
• Respond to stimulus
• Express feelings
• Answer back
• Jerk one's knee, perhaps