Great fun because it's easy, it takes some (but not too much) physical force, and the result tastes great.
Just remember: twice as much flour
as you have fat
Add a drop of vinegar (fill with, say, cheese, ham, parsley, garlic) or add up to 1/4 the flour's weight of sugar (fill with raisins, apple, alcohol, cinnamon).
Keep the ingredients cool and work as fast as you can!
Warming up, the dough turns oily, losing its structure, and after a while it won't bake properly.