We can use simple combinatorics. So recall that proteins consist of 20 different amino acids. So we have 20 different amino acids that our body, as well as other organisms actually use. So that means if the genetic code actually makes sense, then we better have 20 different unique codons that correspond to 20 different unique amino acids that appear in our body. But if we use simple math, we see that if we only have two different positions, two different nucleotides in our codon, then the maximum number of different types of codons is 16. And that's because we have four different possibilities for nucleotides.