<unk>  is the practice of doing mathematical<unk> s using only the human brain, with no help from any computing devices.
Practically,<unk> s are not only helpful when computing tools are not available, but they also can be helpful in situations where it is beneficial to calculate with speed. When a method is much faster than the conventional methods (as taught in school), it may be called a shortcut. Although used to aid or quicken tedious computation, many also practice or create such tricks to impress their peers with their quick<unk> g skills.
Almost all such methods make use of the decimal numeral system. It should be noted that the choice of radix determines what methods to use and also which<unk> s are easier to perform mentally. For example, multiplying or dividing by ten is very easy in decimal (just move the decimal point), whereas multiplying or dividing by sixteen is not; but the opposite happens if one uses the hexadecimal base instead of decimal.
There are many different techniques for performing<unk> s, many of which are specific to a type of problem.
Mental calculation
Visual thinking
Body memory
Muscle memory
Fine motor skill
Visual perception
Mental function
Face perception