Dipper and Mabel talked for a while about it, before agreeing - they would talk to their great uncles, and show them Bill.
It was at the dinner table. Everyone was eating. Even Waddles, snorting away.
"So, um, hey, Great Uncle Ford?" Dipper asked.
"Yes?" The man looked up.
"Mabel and I went into the woods the other day, and we found something incredible - magic actually does exist!" Dipper grinned.
"Yeah! There's this little guy named Bill Cipher who lives in a cave, and he's literally a triangle with an eye!" Mabel supplied, smiling.
Stan and Ford were silent. Until Ford growled, just a little, and said in a dangerous whisper "There is no Bill Cipher."
"But we saw him -" Dipper started.
"There is no Bill Cipher!" Ford exclaimed, slamming his fist into the table, making everything jump.
The kids look a little afraid, wondering about their decision.
"Kids, you should get to bed. And don't bring up magic ever again." Stan said, glaring at them. And so they went into the attic.
"Man, I've... They've never acted like that before." Dipper said softly.
Mabel looked down. "Was it me? Was it something I said?"
"No, no, Mabel, it's just... I don't know what it was, but it wasn't you, I promise..." Dipper held her hand.
"But what if it was?" she asked, tortured by the thought.
Ford stared at his reflection in the plate. "Bill Cipher... Why do I know that name?" he asked, and sighed.
Stan hugged Ford. "Ah, don't worry about it, I'm sure it's nothing important."
"But it is... I know it is. I know it has to be. Why? How?" Ford looked through the window in wonder.