Looking at the number 18, we naturally think "one group of 10, and 8 leftover ones." That's because we use base 10. But think about it carefully — there's no particular reason it has to be grouped by 10s. We're just used to base 10 because we have 10 fingers; grouping by 2s or 5s can represent the exact same count just as well.
Let's try grouping 18 by 5s. Group 18 items into 5s and you get 3 groups with 3 left over. Group those 3 groups (=15 items) into 5s again, and you don't even get one full group — all 3 groups stay as they are. So 18 written in base 5 becomes 33 (the left 3 means "3 groups of five," and the right 3 means "3 leftover ones"). 18 in base 10 and 33 in base 5 are just written differently; they represent the exact same count.
Among the number bases, base 2 is especially important. A computer's internal electronic circuits can only reliably distinguish exactly two states: electricity "on" or "off." So computers represent every number using base 2, with only the two symbols 0 and 1. Even the photos, music, and text you see on screen are all, deep inside the computer, stored as base-2 numbers made entirely of long strings of 0s and 1s.
Understanding number bases also makes the concept of "place value" much clearer. In base 10, the number 253 is 2×100 + 5×10 + 3×1 (each place going leftward is worth 10 times more). Base 5's 33 works the same way: 3×5 + 3×1 = 18. No matter what the base is, the same rule always applies: "each place going leftward is worth that many more times as much."
When studying this with kids, it helps to actually count small objects, like bingo chips or candies, and directly compare how the number of groups changes when you group by 10s versus by 5s. On our activity page, you can change the number of stars yourself and check, step by step, the process of grouping by base 2, base 3, base 5, and base 10 — and see, at the end, exactly how each digit comes together into the final number.