Sister S, who is among us today, had a younger sister whom we shall call "SY". As she was growing up Sister S liked to play tricks on her younger sister SY, so one day, Sister S got her dad's clear plastic office chair mats under his rolling office chairs. Then she placed the mats at the various doorways throughout the house, placing them upside down, so that the pointy spikes that stabilize the mats onto the carpet were now exposed, facing up.
Then Sister S called SY in a very desperate, hurried voice, "SY!! SY!! Come here right now! Hurry!"
Sister SY, thinking that there was something very wrong, ran toward the voice of Sister S, who happened to be all the way at the other end of the house.
Whenever she went through the doorway, she would step on the pointy ends and cry out, "Ow ow ow!!" But because of the adrenaline rush, sister SY just kept on running toward Sister S.
When SY got there, she found Sister S rolling on the ground laughing.