Understanding...

Our English teacher came into the classroom and put a snake on the desk.

As we all crowded around to see it, he said, "This snake will help you to understand that using correct English is very important."

"Is it poisonous?" asked Mary.

"No," he replied.

Mary reached out to stroke it and was immediately bitten. Within seconds, she was spasming and foaming at the mouth.

"However,

Spoiler:

it is venomous," he said.