Board Thread:General Discussion/@comment-37334388-20190301031034/@comment-32182236-20190818162316

@User:We don't know how Chara knew, but we know both Chara and Asriel knew:

"Six, right? We just have to get six... And we'll do it together, right?"

One of the ancient glyphs were written over by Nice Cream Guy to make a list of the flavors. That could have given more information.

But we also know that humans are known for their determination.