It would be "Chess"
On six feet, I am a noxious drink, Of whose effects you shudder to think. Change only my second foot, and then You convert me into a horrible den Where the culprit, who gave the noxious drink, Awaits the fate of which you shudder to think.
Assign every letter of the alphabet its numerical value: A=1, B=2, C=3, and so on, up to z=26.
Can you think of a familiar 7 letter word whose letter values total only 21?
