- a very dirty sock
Take a sock ( an old one preferably one you don't care about getting real muddy), soak the sock in mud, get it real dirty and then put it in a plastic bag. Take it to your class and during your lesson ask someone if they would mind putting on a sock (of course they haven't seen the sock yet), someone will volunteer.
Then take out the muddy sock from the bag and ask them to put it on their clean foot. Hopefully no one will want to do this.
Liken this to the holy spirit not wanting to dwell in an unclean body. How can you put your foot in a dirty sock and expect it to feel ok? You can't ask the spirit to dwell in an unclean body, it absolutely will not.
Alma 7:21 And he doth not dwell in unholy temples; neither can filthiness or anything which is unclean be received into the kingdom of God; therefore I say unto you the time shall come, yea, and it shall be at the last day, that he who is filthy shall remain in his filthiness.
1 Corinthians 3:16 Know ye not that ye are the temple of God, and that the Spirit of God dwelleth in you?