Website retailer ThinkGeek is selling a cotton T-shirt that lights up when it detects a nearby Wi-Fi network.

Known as the Wi-Fi Detector Shirt, it features an animated decal that that has glowing bars which lights up when there is an IEEE 802.11b or 802.11g network in range. However, it cannot yet detect networks using 802.11n Draft 2.0. More bars light up as the signal gets stronger.

The 100 percent cotton T-shirt is only available in black and cost $29.99. The animated decal, which is attached to the shirt with hook-and-loop fasteners, can be removed for washing. A battery pack containing three AAA batteries is concealed in a small pocket sewn inside the shirt.

A ribbon cable that links the batteries to the decal can go through the wash - as long as the shirt is hung up to dry.

Of course, the T-shirt would be a dead giveaway for those illegally using an unsecured Wi-Fi network.