In the binary number system, which is base-2, only two digits are used: 0 and 1. When you add 1 and 1 in binary, you reach a sum that exceeds the highest digit available. Just like in the decimal system where 9 + 1 equals 10, in binary, 1 + 1 equals 10. The result, 10 in binary, represents the decimal number 2. This is because binary counts in powers of 2, so the rightmost digit represents 2^0 (which is 1), and the next digit to the left represents 2^1 (which is 2). Thus, 10 in binary equals 2 in decimal.