of Charlie and Melanie, from the chapter "Siblings' First Date" where the two make a little pillow fort in their playroom and decide to get a little smoochy.
Their dad, as an engineer who designed industrial equipment for various industries, had to go out sometimes to take care of a client or other business matters. In this particular case, their dad had ordered them some dinner for them to enjoy while he was out. The two kits decided to make a little date out of it. Being messy kits, they had to clean each other up after some burgers and shakes, they found themselves kissing each other without knowing it. I was very strange but the two held their kiss and enjoyed it.
This was a big step for Charlie and Melanie, the nature of their relationship was changing in ways they didn't expect. And this evening was showing that there was something a little more than just siblings between the two.