An abacus is an ancient device used to make arithmetic calculations. Before the concept is explained in great detail, ponder upon a few arithmetic questions:

896742 X 843 = ? Or 8888/222 = ?

Hmm.. Heading to retrieve a pen and paper? A calculator? Perhaps even resorting to searching for the answer on the internet?
If we tell you that these equations can be solved without the aid of external devices, you would probably find it humorous and think it is nearly impossible or solving the equation would be a lengthy process.
Now, can you imagine an elementary school student solving a problem that has bewildered your mind? And that too within in seconds and without any external devices, such as a calculator! It may be hard to fathom but it’s true.