#1 (square peg in a round hole) is a great story:

No list would be complete without the ultimate space hack, one which saved the Apollo 13 crew from asphyxiation.

It is 13 April, 9.08pm, 200,000 miles from Earth. The crew of Apollo 13 were two days into their mission, hurtling towards the moon at 25,000 miles per hour, when a loud bang reverberated through their spacecraft.

After surviving the explosion, the crew of Apollo 13 set their vessel on a trajectory to take them around the Moon and back to Earth. The voyage would take four days and they would depend on the lunar lander – docked to the Apollo capsule – for their survival. This space lifeboat would provide power, oxygen and scrub carbon dioxide from the air using lithium hydroxide canisters.

However, the lander was only designed to support two crewmembers for two days on the lunar surface, not three men in space. As a result there were not enough lithium hydroxide canisters on board to keep the air safe. The obvious solution was to use canisters from the capsule – but the ones on the lander fitted into a round hole and the ones on the capsule were square. A design flaw that left mission controllers trying to figure out literally how to fit a square peg into a round hole.

In a backroom in Houston, engineers set to work drawing up a procedure to construct an air-purifier using only the materials the Apollo 13 crew had on board. As carbon dioxide levels rose, the instructions were relayed up to the crew and equipped with two lithium hydroxide canisters, covers from their flight plans, plastics bags, grey sticky tape (almost certainly the same brand that would be later used for the Space Shuttle fly-swatter) and a soggy sock, they set to work.

It took an hour to build the contraption, which ended up looking like a US-style mailbox. As soon the astronauts plugged it in, the carbon dioxide levels began to fall. What’s really impressive is that when they later compared the one built on the ground and the one constructed in space, they looked exactly the same.

So what lessons can we learn from these space hacks? Firstly, that it pays to remain calm under pressure, even when your spacecraft is leaking oxygen into the cold depths of space. Secondly, that the ability to improvise is essential for any astronaut. And, thirdly: make sure you always carry a good supply of sticky tape.