Related Clues:

• Acknowledge a stimulus
• Not just sit there
• Etna does it
• Hit back, say
• Undergo chemical change
• Not keep a poker face
• Duck, say
• What a volcano can do
• Eject violently
• Emulate a geyser
• Gasp, say
• Fight or flee
• Gush, as a volcano
• Get a rash, perhaps
• Flinch, for instance