Como dizer "onde passa um boi, passa uma boiada" em inglês

Inventando:

Where there's a trickle, there's a torrent.

Duvido que eu seja o primeiro.

Easy alliteration, like at http://www.actionagainsthunger.org.uk/m ... o-torrent/.)

A related one, but not a direct translation.

"If you give them the hand they will (soon) take the arm.''

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Now just kidding, "if you make an exception to the rule, the exception is the rule."

And in some different contexts:

If you can do it once, you always can do it.

If you open up the Pandora´s box, there is not going back/it will never be shut again.

And other ways...

Do you want it as a proverb?

If one sheep leaps over the dyke (ditch) all the rest will follow.