The set up was that Watson was looking to interview a nanny. But considering the fact that Watson has no children, the nanny would be taking care of her instead.
As you can probably guess, things only got weirder from there.
The nanny does her best to make sense of what is going on, without much luck. At one point, DeGeneres has Watson go from laughing maniacally to inexplicably crying.
In the end, DeGeneres and Watson fess up to the prank and the nanny takes it in stride, locking Watson up in a big hug.