I found this to be the most confusing for my students. We introduced solid shapes to the pre-k classes; but once they got into Kindergarten they really seemed to get it.
Having "real" items for the students to touch and feel is vital here. Handing them a piece of rectangular notebook paper, then handing them a thick book, helps the FEEL the difference between flat and solid. They can see it, touch it, turn it around...get the full sensory experience between flat and solid.
I spent a lot of time looking for 3-D examples for my students. Here is a list to help you:
Spheres: any ball, orange, globe, gumball
Cubes: dice, square tissue box, block
Cylinders: coffee can, oatmeal canister, wipe container, tennis ball containers, drum, cork, flashlight, telescope
Cones: ice cream sugar cone, party hat, megaphone, orange construction/sports cone
Pyramids: Lincoln Log roofs, decorative stone
Prism: rectangular tissue box, pasta box, juice box, crayon box, toothpaste box
The more examples you can give them of each type of 3-D shape, the better for them to see the differences between same and like shapes...which goes along with K.G.2 and K.G.4 AND they can use these for K.G.5, seeing shapes in the world.
Here is a fun K.G.5 example: