There are many reasons for having clenched teeth...

[plasticjock]

...here's one of them:

John worked in a small chemist but he was not much of a sales assistant. He could never find the item the customer wanted.
The owner had had about enough and warned John that the next sale he missed would be his last.
Then a man came in coughing and he asked John for their best cough syrup. Try as he might John could not find the cough syrup. Remembering his Boss’s warning he sold the man a box of Ex-Lax and told him to take it all at once.
The customer did as John said, walked outside and suddenly slumped against a lamp post.
The Boss had seen the whole thing and came over to ask John what had transpired.
"He wanted something for his cough but I couldn't find the cough syrup so I substituted Ex-Lax and told him to take it all at once" John explained.
"What! Ex-Lax won't cure a cough!" his Boss shouted angrily.
"Course it will" John said, pointing at the man leaning on the lamp post. "Just look at him. He's too afraid to cough!"

[emb123]

excellent! ROFL!