Opportunity knocks but once

Opportunity knocks but once

.Prov. You will only have one chance to do something important or profitable. (You can say opportunity knocks to signal that someone's chance to do something important is here right now.) When Nancy got a scholarship offer from a college far away, her parents encouraged her to go, even though they didn't like the thought of her moving so far from home. "Opportunity knocks but once," they said, "and this may be your only chance to get a good education."
See also: but, knock, once, opportunity
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