Related Clues:

• Closed, curved line
• Closed plane curve
• Curved, closed line
• With hydrogen it forms water
• Nothing, zilch
• Gas, part of the air
• Naught, nowt, zilch
• Nothing, naught
• Regular route for a sentry
• A perfectly round plane figure
• It's in the air - and in water
• Number of people bound together by similar interests - sound round
• The second most common element on earth
• Gas that combines with hydrogen to form water
• Nothing, no value