BC posted a clip ... first from the kitchen -- looks like breakfast -- and then from the gym. You can clearly hear a guy in the background the whole time.
Our first clue it's Rob -- the guy's wearing Yeezys, the official footwear of Kardashian-Jenners everywhere. Also, the bird's-eye view of Chyna's boobs while she's on the elliptical is a dead giveaway. For the record, she's not showing a bit ... yet.
Get it, Rob!