" At the corner of the table sat a stack of papers with a big rock holding them down—made of a material, Lex noted, that she was pretty sure didn’t exist anywhere on the periodic table. Uncle Mort set the rock aside and started to sift through the papers, staring at them intently.
“Should we point out that there’s nothing on them?” Lex whispered to Driggs.
“And spoil the fun of watching an honest-to-God crazy person do what he does best?”
“It’s written in Elixir ink,” Grotton said behind them. When Lex looked at him with the sort of expression that such a statement might elicit, he pursed his lips. “Invisible to everyone but the person who wrote it. Amateurs "

― Gina Damico , Rogue (Croak, #3)