So this duck walks into a pet store and says to the clerk, "Got any duck food?"

"No", says the clerk, "we only sell dog food and cat food." 
"OK", says the duck and walks out. 
The next day the duck walks in the store and says "Uh, got any duck food?" 
The clerk once again replies, "No, like I told you, we only sell cat food and dog food." 
"OK", says the duck and walks out. 
The next day the duck walks in the store and says "Uh, got any duck food?" 
The clerk says "Hey look, I told you two times already that we only sell cat food and dog food!" 
"OK", says the duck and walks out. 
The next day the duck walks in the store and says "Uh, got any duck food?" 
This time the clerk yells "We don't sell any duck food and if you come in here one more time asking, I am going to nail your little webbed feet to the ground!" 
"OK", says the duck and walks out. 
The next day the duck walks in the store and says "Uh, got any nails?" 
"No", says the confused clerk. 
The duck says, "Got any duck food?"