Stories of Gods and Heroes by Thomas Bulfinch

Juno one day perceived it suddenly grow dark, and immediately suspected that her husband had raised a cloud to hide some of his doings that would not bear the light. She brushed away the cloud, and saw her husband on the banks of a glassy river, with a beautiful heifer standing near him. Juno suspected the heifer's form concealed some fair nymph of mortal mould -- as was, indeed the case; for it was Io, the daughter of the river god Inachus, whom Jupiter had been flirting with, and, when he became aware of the approach of his wife, had changed into that form.

Juno joined her husband, and noticing the heifer praised its beauty, and asked whose it was, and of what herd. Jupiter, to stop questions, replied that it was a fresh creation from the earth. Juno asked to have it as a gift. What could Jupiter do? He was loath to give his mistress to his wife; yet how refuse so trifling a present as a simple heifer? He could not, without exciting suspicion; so he consented. The goddess was not yet relieved of her suspicions; so she delivered the heifer to Argus, to be strictly watched.