user18905: I'm building a tic tac toe board for my kids where tapping on a plexiglass surface in any one of the 9 squares would light it up.
What are some ways I can pull off making each of those 9 squares touch sensitive? 
Doesn't need to be anything fancy, but the kicker is that it can't be something attached directly to the plexiglass surface (as that'd obscure the LED light underneath).
Current thinking is buy a sheet of this pressure-sensitive conductive material and cut it in to squares, then somehow transfer pressure form the plexiglass down to the square on the bottom of the box, but I may be overcomplicating it.

user20245: Transparent touch sensitive surfaces are usually based on parasitic capacitance like on an iPhone.  While you can buy loose glass touch surfaces, it is difficult to use such devices if not already attached to an LCD.  The same is true for the much cheaper resistive touch surfaces like on a Newton.  
If you are already using LEDs, I would suggest investigating using the LED to detect the light from the reflection of a finger.  I found people talking about this here, here and here.
I believe this is a still from a video that someone else made using this technique:

For your tic tac toe box I could see touching a location for X's or for O's.  Then touching 1 of 9 boxes you would want to place the symbol in.
Added later...
As @Gerben pointed out there are other options.  I like solving hardware problems in software.  And reversing the potential on an LED then sampling the voltage over time is a snap to implement on many modern embedded processors.  
If you are open to hardware solutions, microchip.com makes a chip which projects an electrical field in front of a PCB for 3 dimensional gesture recognition.  However, the PCB layout is touchy.

Also, there are gesture sensors that (probably) use IR radiation to track movement.  Here's one that supposedly does X Y & Z as well as sense colors.  And it sounds like there is already an Arduino library written for some simple gestures. Looks like there's a YouTube video as well.

18905: <p>I'm building a tic tac toe board for my kids where tapping on a plexiglass surface in any one of the 9 squares would light it up.</p>

<p>What are some ways I can pull off making each of those 9 squares touch sensitive? </p>

<p>Doesn't need to be anything fancy, but the kicker is that it can't be something attached directly to the plexiglass surface (as that'd obscure the LED light underneath).</p>

<p>Current thinking is <a href="https://www.adafruit.com/products/1361" rel="nofollow">buy a sheet of this pressure-sensitive conductive material</a> and cut it in to squares, then somehow transfer pressure form the plexiglass down to the square on the bottom of the box, but I may be overcomplicating it.</p>
 20245: <p>Transparent touch sensitive surfaces are usually based on parasitic capacitance like on an iPhone.  While you can buy loose glass touch surfaces, it is difficult to use such devices if not already attached to an LCD.  The same is true for the much cheaper resistive touch surfaces like on a Newton.  </p>

<p>If you are already using LEDs, I would suggest investigating using the LED to detect the light from the reflection of a finger.  I found people talking about this <a href="http://www.thebox.myzen.co.uk/Workshop/LED_Sensing.html" rel="nofollow noreferrer">here</a>, <a href="http://www.bitartist.org/2008/10/led-used-as-touchinput-sensor.html" rel="nofollow noreferrer">here</a> and <a href="http://makezine.com/projects/make-36-boards/how-to-use-leds-to-detect-light/" rel="nofollow noreferrer">here</a>.</p>

<p>I believe this is a still from a video that someone else made using this technique:</p>

<p><a href="https://i.stack.imgur.com/5gtt1.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5gtt1.jpg" alt="enter image description here"></a></p>

<p>For your tic tac toe box I could see touching a location for X's or for O's.  Then touching 1 of 9 boxes you would want to place the symbol in.</p>

<p>Added later...</p>

<p>As @Gerben pointed out there are other options.  I like solving hardware problems in software.  And reversing the potential on an LED then sampling the voltage over time is a snap to implement on many modern embedded processors.  </p>

<p>If you are open to hardware solutions, microchip.com makes a <a href="http://www.microchip.com/wwwproducts/en/MGC3030" rel="nofollow noreferrer">chip</a> which projects an electrical field in front of a PCB for 3 dimensional gesture recognition.  However, the PCB layout is touchy.</p>

<p><a href="https://i.stack.imgur.com/2NRFu.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2NRFu.jpg" alt="enter image description here"></a></p>

<p>Also, there are gesture sensors that (probably) use IR radiation to track movement.  <a href="https://www.sparkfun.com/products/12787" rel="nofollow noreferrer">Here's one</a> that supposedly does X Y &amp; Z as well as sense colors.  And it sounds like there is already an <a href="https://github.com/sparkfun/APDS-9960_RGB_and_Gesture_Sensor/tree/V_H1.0_L1.4.1" rel="nofollow noreferrer">Arduino library</a> written for some simple gestures. Looks like there's a <a href="https://youtu.be/A3QRyixnEl8" rel="nofollow noreferrer">YouTube video</a> as well.</p>
