how do you put a white person in a jar ? you ca nt , crackers usually come in boxes
The word cracker is a racial slur against white people.  Do you still think it's appropriate in this context?
You should be more offensive or the expression will lose its meaning.
The purpose of a joke is to make people laugh, not to offend them.  Using racial slurs perpetuates feelings of animosity toward people on account of their race, and it doesn't do anything to make your joke funnier.