This is a puzzle that has been tried several times for the students to draw the DNA molecule themselves. It is guaranteed they do it correctly if they follow the instructions carefully.
It is made of complementary parts that only fit each other at specific points.
It is an interesting way to start teaching about DNA in the middle classes like grades 7-9, and a good quick revision method for higher ones before going into more details of the topic.
I prepared it myself more than ten years ago, and every times it proves successful.