This was back in the late 80s when commercials for tinted toilet tank deodorizing cakes and the newest Ziploc bags were big on TV (many of you can probably see this one coming from a mile away).
We were learning about the various human body systems and had worked our way up to the kidneys, bladder, et cetera. One kid in our class decided to ask why urine was yellow (I don’t know if he was serious or not). I don’t remember if our teacher answered him honestly, gave him a bogus answer, or told him to cut it out.
At that point, some wiseacre quoted the tagline that was big in the Ziploc commercials at the time: (in a preschooler’s voice): “Yellow and blue make green!”
Cracked everyone up.