The Grim Adventures of Billy & Mandy

Halls of Time (2x23)

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After missing on a school trip, Grim takes Billy, Mandy and Irwin to the "Halls of Time" for a tour. There, Billy and Irwin mess the hourglasses that symbolize each person's lifespan. When they leave, they accidentally leave their own hourglasses, including Grim's, upside down, resulting in the group getting younger as time passes. In the end, Grim allows the kids to disappear by going before birth, but he can't restore his own hourglass to its correct position and he disappears too.