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To use the Copying Toast, one should press one side of the toast on a book with either text or pictures. The text or picture will then be 'printed' (in the bread's case, 'sticks onto it') on the toast. Eating the toast later will allow the eater to remember whatever content that has been printed on the toast. The user will forget the content (if they never did memorize it normally) if the toasts are expelled out from their system. Like how Nobita had a stomach ache and had to 'expel' them into the toilet, he forgets everything printed on the toasts.

Doraemon Season 1 part 1