I love prototype ideas that use nothing to make something! This is the complete opposite of today's state of things, that is required by the people to use more in order to get less. Here is an excellent idea found in todayifoundout. In order to make a pressure sensor, this guy used a simple static dissipative foam, like the one used when IC chips are shipped. He noticed that this foam has a resistance, which changes according to pressure. This is not an accurate pressure sensor, but it can be used in thousands of applications where great accuracy is not the point. My congratulations! Visit his site for more.