You can't usually tell, but sometimes you can.
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Fri 29 Feb 2008, 11:48,
archived)
The other's a bit more eager, but that one takes longer to catch up- it doesn't move as fast as the other one. If I'd have taken that photo a few seconds later you wouldn't have been able to tell- silly, innit?
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Fri 29 Feb 2008, 11:55,
archived)