Genesis 30:38

And he set the rods which he had pilled before the flocks,.... Opposite them, in the view of them:

in the gutters in the watering troughs, when the flocks came to drink; that is, in places of water, where troughs or vessels were made, into which the water ran convenient for the cattle to drink out of; and here he placed his party coloured rods right over against the flocks:

that they should conceive when they came to drink; as it was most likely they should when they were together at the water, and had refreshed themselves with it; and being "heated" q, as the word signifies, with a desire of copulation, might conceive in sight of the above rods; which were set to move upon their imagination at the time of their conception, in order to produce cattle of different colours; to which no doubt he was directed of God, and it had, through his blessing, the wished for success, as follows:

q יחמנה "incalescebant", Montanus, Vatablus, Drusius; "ut incalescerent", Junius Tremellius.