2. In the decimal system, the smallest natural number whose digits are 0 or 1 and which is divisible by 225 is $\qquad$ .
2. $11,111,111,100$

Solution Note that $225=9 \times 25$, and 9 and 25 are coprime.
For a number whose digits are 0 or 1, if it is divisible by 25, then its last two digits must be 00; if it is divisible by 9, then the sum of its digits must be a multiple of 9, i.e., it must have at least 9 ones. Therefore, $11,111,111,100$ satisfies the condition.