funny story?

We have a writing class on Thursday. Today, teacher passed paper to us that have funny stories in English.

I was thinking about my mistake I made on English.
This is true story.

When I was working in city office, I was working as a receptionist at Optometrist, it happened around 4PM. We ( Optometrist and me) were just waiting a patient who had appointment at 4:30.
Suddenly we heard big bell which is fire alarm and we were told to go outside . I called to patient and told her that she have to cancel an appointment because we didn't know how long takes for this emergency evacuation.

Luckily, we were allowed to back to work after 40 minutes. I send a text to my friend '
We were evacuated because of bum!'

I could imagine the reaction of my friend when she read this message .

Yes, there was a suspect about BOMB!
Now, I never misspell the BOMB to BUM because of this mistake.