Need: One full length straw, and one cut in half.
Take the the full length straw and fold it in half. Crease the fold, so it stays in a "V" shape (figure 1). Now make a "lean-to" using the "V" and the small 1/2 length piece you cut off. Together, they should look somewhat like a pyramid.
Here's the challenge. How can you pick up both pieces of straw using only one other, full-length straw? You can't use your hands, and you have to pick up both straws at once.
Here's how you do it: Insert your "tool straw" under the pinnacle of your lean-to (figure 2). If you push outwards on the folded-over straw, the short straw will fall down onto your tool-straw.
Now slowly pull your tool upwards, and the short straw will lock into the folded over long one (figure 3). Now just lift, and you've done it!