I'm always on the look-out for concepts that are just at/ just beyond M's comprehension level. When I find them, I try to make a game out of them.
Lately, I'd noticed that she tends to mix up the meanings of the prepositions "in" and "on," and I've never heard her say that something is "under" something else. So... I took a medium sized wooden box and we had fun practicing.
First, I gave her a small book and asked her to put it "in" the box. No problem... I asked her to put it on the box. After a little thought, she turned the box and set the book on it. When I asked her to put the book "under" the box, at first she just turned the box a couple more times and put it on the box. I told her "No, you put the book on the box again. Can you put it under the box?" and this time she did!
Once she had the concept, further practice was no pressure- just fun. We tried it with different objects (stuffed animals, etc). Then we changed the box to baby G's bathtub which made it possible to use bigger stuffed animals....AND... M herself. That was the most fun of all! She really got a kick out of getting "under" the tub!