Touch Sensitive LED
Below is the simplest circuit you can get. We can use any NPN. Connect the 220 ohm resistor, LED and transistor as shown in the figure below. Touch the top point with two fingers of one hand and the lower point with fingers of the other hand nad squeeze.
When you squeeze harder, the LED will turn on brighter. We (our body) has resistance and current will flow through our body (fingers) when a voltage is present. the current is amplified by the transistor through your fingers about 200 times and this is enough to illuminate the LED. [Source: talkingelectronics.com]