Coin tower trick: Inertia

STEM @ Home presents: Newton's first law of motion: An object in motion tends to stay in motion and an object at rest tends to stay at rest.
You need a butter knife, a table and a stack of coins.
Stack the coins near the edge of the table, within reach of the butter knife. Swipe the butter knife horizontally at the bottom coin. The stack stays upright but the bottom coin goes shooting off. Why?
You exerted an outside force on the bottom coin, putting in motion, while the stack was untouched so it stays at rest. And don't forget about the friction between all of the coins in the stack.