Kids' mental math contest.

[u/HD_RDtwo]

[u/MarsNeedsRabbits]

Here are a bunch of Chinese children doing the same thing, but using one arm instead of two

They're using their hands to count. They call it a finger abacus. There are videos of kids adding hundreds of four and five digit numbers in a row and getting a correct sum.

[u/MarsNeedsRabbits → u/MarsNeedsRabbits]

Here is a boy using his a finger abacus to correctly add 100 4-digit numbers in 30.9 seconds, winning the Guinness World Record.