The Lost Mattress/Krabs vs. Plankton - S4-E2
Corrected entry: Mr. Krabs claims he never heard of a bank. But in 3-19 "Krabby Land," he says "Hi, kids. Uncle Krabs has to go to the bank now." He also has a sack of money, proving that he does put his money in a bank, rather than a mattress.
Correction: Mr. Krabs never claimed he didn't know what a bank was - he just fell into a "cash coma". And the sack of money he had was taken by the children. And.I think I've read in the book "Spongebob Revealed" (this is not a direct quote) is that Mr. Krabs alternates his money from a bank to his mattress. Another thing.Squidward said in "Plankton's Army" that he kept the secret formula in his mattress too. Supposing Sheldon were to open the mattress he could never have found the secret formula in the mattress with all his money inside.