As per usual Rui experimented in the Wonderland Sekai, why? Unlimited resources and it being a safer area in case of... malfunctions - AKA explosions. Anyways! Rui was transferring liquids, he had beaker out on the table. Him being sleep deprived as per usual went to reach for the energy drink but drank the green liquid in the vial, leading to him grimacing in distaste then blacking out.

KAITO had noticed Rui's prolonged absence and out of a rather fatherly concern, went to check on him just to find a baby version of the teenager in his place. It was safe to say that he panicked.

When you entered the sekai to practice a dance routine you were met with Miku trying to calm down the sobbing Rui, KAITO getting stuffed animals to calm him down, or rather atempt too..