The plastic must also be flat and smooth for the reading to be accurate, and the object must be brought to the machine (which is not very portable) for analysis, which takes yet more time.

ECONOMIST: Recycling

The mariner uses a 250 watt free-spinning motor mounted in the front wheel for propulsion and high capacity 36 volt 9 amp hour advanced lithium battery (LiFEPO4) which provide a smooth, powered ride on flat and hilly terrain.

FORBES: Green Gear Test - Folding Electric Bikes

Over the centuries, the river had slowly carried away mountainsides, tumbling and cracking rock, reducing boulders into a rainbow of smooth, flat stones the size of a child's hand.

CNN: The seven stones of simpler living

Almost everyone seems to love Cathay Pacific for its smooth service and fantastic Hong Kong hub, but with an 81-inch-long first class bed, flat beds in business class, and shell seats in economy (meaning that even if the person in front of you reclines, their chair-back does not), Cathay's a particularly comfortable option for those who value their personal space.

BBC: Smooth skies: The world's best airlines