A small force sensitive resistor with a 0.16" (4 mm) diameter active sensing area.
This FSR from Interlink Electronics will vary its resistance depending on how much pressure is being applied to the sensing area. The harder the force, the lower the resistance. When no pressure is being applied to the FSR, its resistance will be larger than 1MΩ, with full pressure applied the resistance will be 2.5kΩ.
Two pins extend from the bottom of the sensor with 0.1" pitch making it bread board friendly.
These sensors are simple to set up and great for sensing pressure, but they aren’t incredibly accurate. Use them to sense if it’s being squeezed, but you may not want to use it as a scale.