The transition from an egg to a developing embryo is one of life’s most remarkable transformations. Yet little is known about it. Now Whitehead Institute researchers have deciphered how one aspect—control of the all-important translation of messenger RNAs (mRNAs) into proteins—switches as the egg becomes an embryo. That shift is controlled by a beautiful mechanism, which is triggered at a precise moment in development and automatically shuts itself off after a narrow window of 20 to 90 minutes.