In a vacuum; if two identical objects with the same mass are dropped from the same height at the same time, they will hit the ground at the same time. However, when comparing two objects in the air; such as a sheet of paper versus a ball of paper; the crumpled ball will take longer to reach the ground than the flat piece of paper because it has less surface area, and therefore has less air resistance.
Gravity pulls all objects towards the ground and air resistance, which depends on the shape of an object, pushes up against them, slowing them down. Eventually an object will fall so fast that the force of gravity on its mass balances the air resistance and it stops slowing down and falls to the ground. This is called terminal velocity.
A paper plane, for example, moves forwards because of a force that pushes against it, but the force of air pushing against the wings slows it down (this is a force called drag). Changing how an object flies by altering what forces are acting on it can make it change direction or speed or both.
To test this, try dropping a piece of paper and a piece of paper that is scrunched up into a ball at the same time from legitimately the same height and see which one hits the ground first. This will help students notice that mass is not the only factor that affects how things fall, and it shows why changing the shape of an object can have a large effect on its behavior.
