Jacob chimes in

"What are you testing?" Jacob added.
"Look, it's hard to explain, and it doesn't really matter. I just need you to trust that I need to go for a while, but I will be back."
"No, that's not good enough! I need to know why." Rebecca insisted.
"Fine. I'm testing a device that allows me to transform into other people. I know it works, but I need to make sure it lasts, that it will work for a long time." Walter explained, annoyed.
"Bullshit." Jacob said
"You don't believe me? Fine. I'll prove it to you." Walter said.