Swap By Failure is designed as a one-page pen and paper that has all necessary rules to play it on just one page. For this game, there is no general setting, but I recommend a normal non-magical world on the height of our current civilization and normal everyday people as player characters.

What is needed to play:
- pen and paper
- at least one ten-sided die/dice
- one GM (game master) and three plus players
- a pinch of imagination

Character creation:
First, draw a raster with six rows and as many columns as players plus one on an empty sheet of paper. In the upper left corner write your characters name. Below the name write five character-defining skills your character has. In the second column write beside each skill how good the character is using this skill. However, only distribute thirty points between all skills. Also, each skill must have at least two points and can't exceed eight points.
Now it is time to fill out the top row. Besides your character name write in the next column your name. Then clockwise write down the names of each other player. Not the game master as he or she has to sit this one out.
The next step is to fill each empty box of the raster with a two. Why will be explained later.
As the last step define five aspects your character has. Those can be personal quirks or past achievements.

Caroline wants to name her character "Anna" so she writes it in the top left. As she wants to play a young up and coming journalist, she writes down the following skills below the name: researching facts; writing; socializing; memory recall; urban camouflage.
Next, she distributes the skill points. Eight go into writing and six into researching facts. That leaves her with sixteen points for the other three skills. She puts further six points into socializing and six points into memory recall. Leaving four points for urban camouflage. The ability to blend in with crowds of people.
After writing down the player names and filling the rest of the boxes with two's she thinks about the aspects of her character. "English Lit. major" fits in well with the character's job. "Former cheerleader" and "typical middle-child" help further flush out her past. To give her character some personal touch she chooses "sweet tooth" and "stubborn". With this step completed Caroline has her character ready to play.

Roll for success:
If the characters face a challenge they either depend on one of their skills or have to risk an uneducated roll (against one). To succeed they roll the ten-sided die and try to match the value of the skill or go below. The higher the value of a skill the easier it is to roll it or roll less. If a person doesn't have a corresponding skill they roll against a value of one. However, a role can be made easier by invoking a relatable aspect. More to that point later.

Swap At Failure:
The title of the game so this mechanic must be special. Indeed it has ramifications for every player in the game. If a character fails a significant challenge every player hands clockwise their current character to the next player. In the game, the souls of the characters have just swapped bodies! The in-game reason why is up to the game master to decide. Once the characters have swapped bodies they notice that their access to their original skills is rather disturbed. On the other hand, each character/player has a strange connection to their new body. This is reflected on the character sheet in the column under each player name. Every skill is at two. Meaning they can access part of a skill, but not the full extent.

Invoking aspects:
Rolling a two or below is rather hard with a ten-sided die. To help them out players can evoke aspects of either their original character or their current one. As long as these aspects a related to the challenge. They also can invoke skills of their original character as aspects. Each invoked aspects adds plus one to the challenged skill, but only a maximum of two aspects can be invoked. It is technically possible to roll against a ten in a skill challenge provided the original skill is eight and two aspects are invoked. This would result in an unfailable situation. To avoid this rolling a ten always means failure.

Caroline now has the character sheet of Brandon, an environmental biologist. After the swap and the first confusion, the players decide their characters have to pretend nothing is wrong and live each other's lives for now. That includes each other's jobs. Caroline has to finish an environmental study as Brandon. She has limited access to Brandon's skills. She chooses biology for a base of two. Looking at Brandon's aspects she chooses "always prepared" as surely he must have some notes around to help his work. However, the other aspects are not helping. In the last moment, she remembers that she can invoke aspects or skills of her original character. The aspects aren't helping, but she chooses to invoke the skill "researching facts" as an additional aspect. With two invoked aspects she has to roll against four ( base two plus one for each invoked aspect). Rolling the dice she gets a four. That means she passed the roll barely. It also means that she would have failed if she hadn't remembered that she could invoke skills of her original character as an aspect.

Getting used to a different body:
Of course, the longer one controls a body the familiar this body gets. To reflect this each time a player succeeds in a significant challenge they can raise the appropriate skill in their column by one. However, this way they can't get over the highest skill-point value in a row.

Long-term survival in a different body:
For each day a player controls a character - that is not their original and the day isn't interrupted by a swap - they can raise one skill by one point provided it does not exceed the highest value in a row.
Alternatively, they can attempt to learn a new skill or raise a skill that is currently the highest in the row. For the attempt, they have to roll a ten-sided die against two plus up to two related aspects. As this roll has no significant outcome for the characters no swap has to be made on a failure.

Martin currently controls "Anna", the character Caroline originally made. While playing he notices that "Anna" is being followed by unsavory persons. He tries to shake them by using the "urban camouflage" skill. He used it before to success and managed to raise the skill to three. With the help of aspects, he manages to let "Anna" slip past the unsavory people undetected. As a reward he raises the skill to four, matching the original skill-point value of Caroline's column.
At the end of the session, the gamemaster declares they made a day without a bodyswap. This allows the players to raise a skill. Martin likes the "urban camouflage" skill but he can't raise it automatically as it is at the highest in the row. Instead, he decides to train it by rolling the die. To his luck, he succeeds and raises the skill to five.
In the next session, someone fails a roll and the characters swap. Caroline gets back her original character "Anna". Despite being her original character Caroline can only use the "urban camouflage" skill with a value of four. However, the next time a day passes without a swap she could raise the skill to five without needing to roll for it.

When the game ends:
How the game ends is up to the gamemaster. Maybe the characters accomplish a task or survive X given days. Will it end with each player in control of their original character?