The Standard Book of Spells
Undetectable Extension Charm
When cast on a container object (such as a bag, flask or bottle), the capacity of said object is increased, without changing the exterior appearance. Muggles do not understand the concept of this spell fully, but it has been hinted at in their media. For example, a character known as ‘The Doctor’ travels in a box which has the Undetectable Extension Charm placed upon it, making it look bigger on the inside.