user19700: here is a very simple circuit, with a LED between pin 2 and ground, and a loose wire in pin 8.

Here is the sketch that was intended for use with a small 4-contact switch like these in every arduino starter kit :
int yellowCIn = 8;
int yellowCOut = 2;

void setup() {
  pinMode(yellowCIn, INPUT);
  pinMode(yellowCOut, OUTPUT);
}

void loop() {
  if (digitalRead(yellowCIn) == HIGH) {
  digitalWrite(yellowCOut, HIGH);
  }
  else {
    digitalWrite(yellowCOut, LOW);
  }
}

However, simply moving my hand near the loose white wire is enough to light the LED, moving it away to put it off. As I am writing this, with the arduino connected by usb, rather than on the blue battery visible in the picture, I notice moving my hands away from the keyboard lights the led, moving them close to the keyboard powers it off...
So back to my title : are my hands magic, my electronics competency crap (well, it is, for sure) enough that I never heard of such a tremendous static electricity effect (and never fried one of the many computers I built from parts), or does this board have dire problems?
Thanks for your attention :)

user10135: Try configuring your input pin as INPUT_PULLUP instead. This connects an internal pullup resistor to stop the input value from drifting around.
I would only use the "INPUT" mode on its own if I have that pin wired to something on the board and the levels won't be erratic.
Your hands are magic - they pick up interference from various sources and the high impedance input of the chip pick this up...
(There's a bit more on the modes here: Arduino DigitalPins page)

user19017: Nothing's broken; you have discovered  the nature of a floating pin.  Unfortunately, that means that strange things happen...
A very small charge from your hand is triggering the logic because the state of the pin (high or low) is not set.  To fix this, you need to add a pull-up or pull-down resistor (or activate the internal input-pullup resistor).
You can read about how to accomplish this here.

19700: <p>here is a very simple circuit, with a LED between pin 2 and ground, and a loose wire in pin 8.</p>

<p><a href="https://i.stack.imgur.com/RGims.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RGims.jpg" alt="enter image description here"></a></p>

<p>Here is the sketch that was intended for use with a small 4-contact switch like these in every arduino starter kit :</p>

<pre><code>int yellowCIn = 8;
int yellowCOut = 2;

void setup() {
  pinMode(yellowCIn, INPUT);
  pinMode(yellowCOut, OUTPUT);
}

void loop() {
  if (digitalRead(yellowCIn) == HIGH) {
  digitalWrite(yellowCOut, HIGH);
  }
  else {
    digitalWrite(yellowCOut, LOW);
  }
}
</code></pre>

<p>However, simply moving my hand near the loose white wire is enough to light the LED, moving it away to put it off. As I am writing this, with the arduino connected by usb, rather than on the blue battery visible in the picture, I notice moving my hands away from the keyboard lights the led, moving them close to the keyboard powers it off...</p>

<p>So back to my title : are my hands magic, my electronics competency crap (well, it is, for sure) enough that I never heard of such a tremendous static electricity effect (and never fried one of the many computers I built from parts), or does this board have dire problems?</p>

<p>Thanks for your attention :)</p>
 10135: <p>Try configuring your input pin as INPUT_PULLUP instead. This connects an internal pullup resistor to stop the input value from drifting around.</p>

<p>I would only use the "INPUT" mode on its own if I have that pin wired to something on the board and the levels won't be erratic.</p>

<p>Your hands are magic - they pick up interference from various sources and the high impedance input of the chip pick this up...</p>

<p>(There's a bit more on the modes here: <a href="https://www.arduino.cc/en/Tutorial/DigitalPins" rel="nofollow">Arduino DigitalPins page</a>)</p>
 19017: <p>Nothing's broken; you have discovered  the nature of a floating pin.  Unfortunately, that means that strange things happen...</p>

<p>A very small charge from your hand is triggering the logic because the state of the pin (high or low) is not set.  To fix this, you need to add a <a href="http://playground.arduino.cc/CommonTopics/PullUpDownResistor" rel="nofollow">pull-up or pull-down resistor</a> (or activate the internal <a href="https://www.arduino.cc/en/Reference/PinMode" rel="nofollow">input-pullup resistor</a>).</p>

<p>You can read about how to accomplish this <a href="https://learn.sparkfun.com/tutorials/pull-up-resistors" rel="nofollow">here</a>.</p>
