Max and Tony have to create a 3D model of a body system for their science exhibition. Max wants to do the circulatory system whereas Tony wants to do the skeletal system. They spent the whole day arguing. The next day they didn't talk and each decided to chat with other friends. The third day, they decided to do flip a coin to choose a body system and Tony won. They wanted to work, but they didn't have any materials to create the skeletal system, so they played computer games. The last day, they started to work, but they couldn't finish. What was the cause they didn't have any materials to do the skeletal system?