Abstract In nucleate boiling the sources of vapor nuclei are important. One source arises when a drop of liquid resulting from bursting vapor bubbles strikes the liquid surface and entrains bubbles. This is called secondary nucleation. This research studied differences in bubble entrainment, even for successive drops for water drops falling through air. Close-up photographs were taken soon after impact of the breakup of an air film trapped upon drop impact. They show two distinct patterns. In one pattern, an uniform distribution of small bubbles marks the interface. In the other pattern, bubbles penetrate both the drop and pool at one location on the interface. The interpretation from many photographs is that the first pattern results from a surface disturbance due to a Rayleigh-Taylor instability. The second pattern results from the air film being pinched to a point and then the confined air squirts into both the drop and pool.