It means you could get two things done at one time or one
trip.
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Technically yes. You could either use a very large stone that would kill two birds (or more) at one time; else you could fire or throw the stone ensuring that it has enough velocity to kill first one bird and then the second.
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They pick up a stone than because the birds are so close they
throw it and it hits them so sad
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Well, maybe its a different way of saying "kill two birds with
one stone"...