As I was thinking of how it might be explained to someone ...
Typically we've thought of conception as the UPS truck driving up to Mrs. Egg's house, and the driver ringing the doorbell and delivering the package of DNA, and then from that point on the truck and driver are jettisoned from our thoughts.
Now it appears that might not quite be the case; it seems as if the driver himself may enter into the house for a little hanky-panky with Mrs. Egg, leaving behind a bit more than the package he was tasked to deliver.
(Now, my friends, explain to me why we guys always have to turn discussions of sex into sex-related discussions. Disgusting, we are. Or maybe it's just me ....)