Infinity is strange and confused mathematicians to start with. It comes up naturally when we first learn that counting goes on forever. To help us understand the strange behavior of infinity, mathematicians went back to the basic and tried to define what it means to count.
Imagine you have a bag full of sweets. You cannot see in the bag, but you want to know how many sweets you have. The best way to do that would be to reach in the bag, pull out the first sweet and say "one." Next, you would pull out the second sweet and say "two." You would keep doing this until the bag is empty. The last number you say is how many sweets you have in your bag. This is how we learn to count, no matter what objects you have in your bag.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2018.00061