If hydrogen were used as a crack detector (it is not because of the fire hazard), it would escape four times faster.

ECONOMIST: Tech.view

During our visit the soundtrack of war was playing in the hills around Sherwangai - the dull crack and thump of mortars, punctuated by bursts of machinegun fire.

BBC: Pakistan's hidden war

An armoured bus belonging to a crack police unit, the anti-crime task-force, was set on fire.

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