At the Olympics men don't run against women, children don't (usually) swim against adults, and heavyweight boxers don't punch their featherweight counterparts.
In all sport, we categorise humans according to their physical attributes; it is just a bit more detailed at the Paralympics.
Here, athletes are grouped according to their disability.
In the broadest terms, that boils down to three main categories: athletes with a physical impairment, athletes with a visual impairment, and athletes with an intellectual impairment.
From this general starting point, each sport creates its own rules and classifications, which vary widely.
As with all sports, athletics gives its categories numbers, which look complicated but actually make things simpler to follow.
The lower the number, the more severe the impairment of athletes:
- 11-13 are for athletes with visual impairments
- 20 is for athletes with intellectual impairments, who all compete together
- 31-38 are for athletes with cerebral palsy
- 40-46 are for those with loss of limbs or limb deficiencies
- 51-58 are for wheelchair athletes
Athletics uses letters too - T for track events, F for field. They combine these with the numbers above to give a competition a code.
So for example, wheelchair racer and double gold-medallist David Weir will compete in T54 races. That means track events for athletes who use a wheelchair and who are middle of the scale when it comes to the level of their impairment.
Once you have classified impairments, the tough bit is to measure them.
Officials assess athletes very carefully before events. But there is always controversy, argument, and sometimes even cheating.
For example, next Thursday Oscar Pistorius will look to retain his 100m crown. As he lines up his two prosthetic blades in the blocks, next to him British hopeful Jonnie Peacock will run using just one.
In that race, double amputees and single amputees compete side by side, which might not always seem fair. That rule is the choice of officials, not athletes.
But there have been cases too where competitors have attempted to manipulate the Games. The most famous came in 2000 when 10 members of the Spanish wheelchair basketball team had to hand back their medals because they were found to have faked an intellectual impairment.
But all competitions are vulnerable to cheating. And as with doping at the Olympics, there are trained professionals to catch anyone attempting to grab a gold unfairly.
Paralympic classifications may be complex, but they are the only way to make these Games purely and simply about sport.