Conrad struggles to understand scientific explanations that his older brother is just beginning to get. Why do some bath toys sink while others float? Surface tension versus gravity. Why do birds need to flap their wings? Gravity. What is Conrad testing when he throws his cheese cubes on the floor? Gravity.
Most children have difficulty with that V phoneme at age two, so it's not surprising that Conrad has substituted a B, but he makes a grabbing gesture with his hands when he asks me about grabbity. I think he's trying to figure out what's doing all this grabbing.