Yet, all the while, his keen brain held a plan.

He had kept the real identities within his own keen brain.

But behind those closed eyelids, a keen brain was at work.

His keen brain had sensed the situation in a moment.

His keen brain, usually alert, was working slowly for the moment.

The Shadow, his keen brain at work, had found an answer to a problem.

They could almost feel the master strategist's keen brain at work.

His keen brain had been wrestling with this problem for hours.

He was reported at the time to have had a keen, mischievous political brain.

But behind that smile, a keen brain was considering another theory.