Force Sensitive Resistor from Sparkfun

Pressure-Sensitive Conductive Sheet from Adafruit

A flex sensor measures how hard something is pressing (force) on the surface of the sensor.

Technical Description: 
The pressure sensor is a piezoresister, which measures mechanical stress. This means that the sensor measures resistance between the surfaces. As someone applies stress (force) on the surface, the resistance changes.

Limitations:
Depending on the sensor, the amount of resistance may vary, as well as the maximum amount of pressure that can be applied.

Skill:
Easy

Sample Sensor:
ForceFlex:
https://www.sparkfun.com/products/11207

Conductive Sheet:
https://www.adafruit.com/product/1361?gclid=Cj0KCQiAnY_jBRDdARIsAIEqpJ1rrf-zJR5xmdydou6NXkTCSGLpBP51cR_Tw-I5B12KOiwykCgHmdUaAjHeEALw_wcB

Sample Exercise:
http://bildr.org/2012/11/flexiforce-arduino/