I have a fairy by my side 
Which says I must not sleep, 
When once in pain I loudly cried 
It said "You must not weep" 
If, full of mirth, I smile and grin, 
It says "You must not laugh" 
When once I wished to drink some gin 
It said "You must not quaff". 

When once a meal I wished to taste 
It said "You must not bite" 
When to the wars I went in haste 
It said "You must not fight". 

"What may I do?" at length I cried, 
Tired of the painful task. 
The fairy quietly replied, 
And said "You must not ask". 

Moral: "You mustn't."