This is my favorite way of distinguishing between complementary and supplementary angles. First, we know that complementary angles are two angles whose sum is 90 degrees. Likewise, supplementary angles are two angles whose sum is 180 degrees.
First ask what name comes to mind when you hear the word complementary…complement. Ask who can you give a complement to? Can you give a complement to your grandmother on her 90th birthday? What about on her 180th birthday?
Perfect! So, you can only give complements to 90 year old grandmothers, thus, complementary angles can only by 90 degree angles.