It might be the preserve of fantastical action movies, but we've probably all wondered at some point or other if it's actually possible to fly through the air by firing bullets down at the ground.
The principle here is pretty simple. If you fire a bullet forward, the recoil pushes you back. So if you fire downward, the recoil should push you up.
But can it be done? The short answer is, amazingly: yes. The longer answer is of course a little more involved, but amazingly interesting, combining guns with rocket science. For instance:
[O]ur optimal craft comprises a large number of AK-47s (a minimum of 25 but ideally at least 300) carrying 250 rounds of ammunition each. The largest versions of this craft could accelerate upward to vertical speeds approaching 100 metres per second, climbing over half a kilometre into the air.