I still remember when I was in primary school and about 8 years old some other kid came up with that showerhead nonsense in class and I replied that's wrong and never happened. No gas ever came out of any showers. That's not how Zyklon B works, you gotta throw the pellets in from above and wait for the body heat to release the HCN from the adsorbent. Grandpa knew, I knew. 

mfw I learned quite interesting stuff from my grandparents as a little boy