I bought Froot Loops (or whatever brand that has the same colored loopy cereal), pretzel sticks, mini marshmallows, and peanut butter. On a poster, I glued a piece of cereal of each color on it and then labeled each one as an element. For example, blue ones are oxygen, red is hydrogen, etc. Now I was prepared to teach my lesson.
We would discuss what a molecule is and how different ones are formed. By this time, we have already discussed elements and periodic table. I also use this alphabetical list of common molecules for a reference.
Then I demonstrate making a water molecule by putting a red one on a marshmallow by using the peanut butter as glue. I do the same for another red one and a blue one. After all three are completed, I attach them together by sticking the pretzel stick into the marshmallow. Now the students have a 3D view of a molecule which really stands out in their minds. I give the students a list of molecules to make and a large piece of paper to place them on. For students who finish early, they may make their own creations but they have to write label it on the paper. When everyone is done, we look at all of the completed molecules to see if we can identify them just by looking at them. After the lesson is completed, the students may eat their creations.
My students have really enjoyed this lesson and I have done this on the elementary level and the high school level. Students did tend to want to eat the parts before the lesson so I actually had to allot them a certain number of marshmallows and pretzels. I think this is a fun way to learn molecules and students enjoy it enough to remember the concepts so it is pretty successful.