It was probably quite easy to pull the books apart when only a few pages were interlaced. As you weave in more and more pages, it becomes surprisingly difficult to pull them apart - until they became impossible to separate! The reason for that is friction.  

You experience friction every day. When you walk, there is friction that prevents your feet from sliding on the floor. Imagine if every surface was as smooth as ice - it would be very difficult to get around! Friction is a type of force, or a "push" or a "pull", that can act on objects.

In physics, the force of friction between two surfaces is determined by something called the coefficient of friction. A higher coefficient of friction means that there will be more friction between two surfaces, or they will be less "smooth." For example, the rubber soles of your shoes and a carpeted floor have a much higher coefficient than ice skates. The amount of friction also depends on something called the normal force. In this context, "normal" means "perpendicular to the surface." A higher normal force will give higher friction. For example, imagine two identical cardboard boxes sitting on a wooden floor. One box is empty and very light, and one is full of things and very heavy. In both cases, the coefficient of friction (between cardboard and wood) is the same. However, the heavier box will be much more difficult to push across the floor. This is because there is a higher normal force between the box and the floor, so there is more friction.

It turns out that when you try to pull the two books apart, the interlaced pages in the middle are squeezed together more tightly - this increases the normal force between them, thus increasing the overall friction. This means that the more you pull, the higher the friction, and the harder the sides are to pull apart!

Although the classic demonstration of this project uses large phone books, it works well even for much smaller volumes, including sticky notes.