This is a harmless prank. It is simply taking binders, books, etc. out of a backpack and turning it inside out. Commonly known as "nugging" or "turtling" this is a hilarious prank to try on anyone.
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Step 1: Preparation
Almost all bags can be turned inside out or are "nuggable". The easiest are backpacks that many people bring to school with 2 shoulder straps and a soft back. Backpacks made by L.L. Bean are the easies of all because they lack rigidity in structure. Find a secluded spot where no one can see you because, chances are the owner won't be very happy about what you are doing.
Step 2: Remove
Remove all binders, books, calculators, pencil cases and bulky items that will get in the way.
Make sure you don't lose anything.
Step 3: Flip It
When all of the persons belongings are removed, grab the bottom of the bag and pull upwards. Then tuck in shoulder straps and other objects. This should turn most bags inside out.
Step 4: Finish
Put the binders, books, etc. back into the backpack and zip it up so it is almost like an egg.
For added effect put the backpack somewhere interesting like, the top of a vending machine.
Step 5: Laugh
If all goes as the planned, the person will see their bag and experience a slight bout of anger. After that, it's laughs all around.