You silently enjoy the gentle bumps of the bus causing Kat’s breasts to bounce inside you, and focus your thoughts on milk. You imagine it flowing, pouring, and pooling inside you, letting the taste of it fill your mind. It works, and within seconds, you can feel her breasts becoming ever so slightly heavier as Kat’s tits start to produce the creamy liquid. Above you, Kat licks her lips and seems to be lost in some pleasant thought, which seems to be a good sign that she won’t put up a fight. You picture gallons and gallons of the milk, the trickle in your mind’s eye increasing into a torrent. Before long, Kat’s milk jugs have swollen perky and round, firm and tight despite not having grown enough for her to suspect anything. Having succeeded in step one, you switch gears and think about Kat’s nipples swelling, making them plump and juicy and grabbable. You subconsciously try to suckle at her nipples with your mouth, and despite not having one, it seems to have the intended effect. Her nipples erect inside you, and start slowly increasing in size. You picture wide, sensitive areolas pushing their pointy nubs proudly outwards, and they swell enough that you start to jut outwards. Kat, noticing the increased pleasure of her nipples, looks down to inspect you.
“Ooh..... you’d better not be up to any funny stuff...” she threatens, adjusting your straps. The changes to her tits seem to have been subtle enough that she hasn’t noticed her rounder breasts and pointier nipples. Two teenage boys sitting nearby watch closely and nudge each other gleefully, having paid slightly more attention.