A prime number can only be divided exactly by 1 and itself. Sieve the 100 grid to find them all, then test some yourself.
Tap 2, the first prime. All its multiples will drop out.
A prime has exactly two factors: 1 and itself.
Share its dots into 2, 3, 4… rows: for a prime there is always some left over.
1 isn't prime: it has only one factor.
2 is the only even prime.
A number with exactly two factors: 1 and itself. 7 is prime because 7 dots only make one rectangle, a single row. 8 is not, because 8 dots also make 2 rows of 4.
No. A prime needs exactly two different factors, and 1 only has one. So the primes start at 2, which is also the only even prime.
A way to find primes on a 100 grid. Circle 2 and cross out its multiples, then circle the next number left (3) and cross out its multiples, then 5, then 7. Every number still left is prime: there are 25 primes up to 100.
Children usually meet prime numbers at about 9 to 10, once they know factors, and learn the primes up to 100 and why 1 is not prime at about 10 to 11.