Possible Clues:

• With us it's the limit for them to be so impulsive
• Things provoking reactions
• Invoke response? We do turn litmus a shade of indigo
• Things promoting activity
• Actions producing a response
• Rousing influences
• Action preceders
• Things that produce a response
• Incentives limit us, strangely
• Spurs number one on list for a transfer, Man United initially involved