Six-fold degeneracy of Leucine codons

The genetic code for leucine (Leu) can be either CUN or UUR. Consequently, either UUA or CUA, as well as either UUG or CUG code for Leu. That is, these first codon position changes do not alter the amino acid, and are the only exceptions to the rule that first positions 'always' lead to amino acid substitutions in the Universal Code.

Homework: There are two other amino acids encoded by six codons each: find the, and explain why they don't violate the rule.