Object lesson: Every choice we make (each stick, each lego) affects our destiny/future/testimony. 
(by Kristin in UT)

I have 2 small drawstring bags, one filled with Legos and one filled with junk (broken popsicle sticks, small rocks, broken hunks of concrete, twigs, etc.)

I have 2 children each come to the front of the room, in view of all, and build a house out of their materials, so we can then compare the different "houses".

If we make a rash of good choices (legos), we have a strong, healthy, safe future and life. If we make poor choices (which lead to less alternatives and more poor choices), then we end up with a shaky future that won't take more than one shake to send it toppling to the ground.