I dirtied the glass by misting it with water and sprinkling it with dust. I placed the glass on an easel on the table.
At the appropriate time in the lesson, I used a glass cleaner wipe to clear away a swath on the glass.
The lesson was to visualize the filth (spotting) of the world and accumulation of sin throughout the week. By coming to church and partaking of the spirit and the Sacrament,renewing ourselves (becoming unspotted) we can clean our glass and keep a clear eternal perspective. The dirty side represented our side and the clean side was that of the Lord's. The glass is always clear, we just need to do our part to clean it up and keep a clear eternal perspective.
D&C 59:9 "And that thou mayest more fully keep thyself unspotted from the world, thou shalt go to the house of prayer and offer up thy sacraments upon my holy day;"