It all happened by accident. This was back when they were in high school and I was sitting on the couch, yellow marker in hand, and doing a seek-a-word puzzle. One of the kids came in, asked what I was doing, then the other so I gave my son and daughter each a different colored highlighter than mine and said "okay, I will put this puzzle on the coffee table and we will make a game out of it and whoever sees the word first marks it with their color".
That worked for about 2 minutes before one of us accidentally marked the other on the leg with the marker. Before you knew it we had totally forgotten the puzzle and were chasing each other around writing on each other with our highlighters!
By the next day when the kids went to school they still had a few of the highlighter streaks on them and had to explain to their classmates what we had done. :-)