I see a lid with a slit in it.
Can you fit a pit in the slit?
Try to slip the pit into the slit.
See if the pit will fit. 
Will the pit fit in the slit?
I bet it will not fit! 
 
Now try to fit words in the slit.
Can you fit the word sit in the slit?
Can you fit the word kit in the slit?
Will all the words fit?
I bet all the words will fit in the slit!