### Theory:

Eratosthenes was a Greek mathematician, astronomer, and geographer in Egypt in $$200$$ B.C. He invented a method for finding prime numbers that are still used today.

Let's see how to find the prime number using the Eratosthenes sieve method:

Step 1: Cross out $$1$$, since it is neither prime nor a composite number.

Step 2: Encircle $$2$$. All the numbers divisible by $$2$$ are even numbers. So cross out all the multiples of $$2$$.

Step 3: Encircle $$3$$; cross out multiples of $$3$$.

Step 4: Encircle $$5$$; cross out multiples of $$5$$.

Step 5: Encircle $$7$$; cross out multiples of $$7$$.

Step 6: Encircle $$11$$; cross out multiples of $$11$$.

Step 7: All the encircled numbers are prime numbers. Rest all the crossed-out numbers except $$1$$ are composite numbers.

$$2$$, $$3$$, $$5$$, $$7$$, $$11$$, $$13$$, $$17$$, $$19$$, $$23$$, $$29$$, $$31$$, $$37$$, $$41$$, $$43$$, $$47$$, $$53$$, $$59$$, $$61$$, $$67$$, $$71$$, $$73$$, $$79$$, $$83$$, $$89$$, $$97$$.

Yes! we got all prime numbers from $$1$$ to $$100$$.