See-Thru Glasses (シースルーガラス) are a pair of glasses invented by Senbei Norimaki that make clothing transparent so the person wearing them can see any clothed person naked. It also makes ground, water, buildings and objects transparent as well.
Senbei invented the See-Thru Glasses because Arale Norimaki told him that she was missing a certain part the other girls had that she did not, which he thought was their crotch. Senbei made the glasses to see one and make it so she had one. While taking it outside to look for "study material" he found Aoi sitting on a park bench reading a book. He put the glasses on to see her naked, but a cat was sitting on her lap, blocking her crotch from being seen. When he heard Midori was coming by on her scooter, he went to go look at her, but was hit by the scooter, breaking the glasses, since Senbei went to look at the front.
Later on in the manga, Tsuruten Tsun is looking for a 5 millimeter thick metal plate, so he goes to Senbei's house for supplies. Senbei brings Tsuruten to his laboratory, and Tsuruten finds the See-Thru Glasses. Senbei won't let Tsuruten have it, but then Tsuruten persuades Senbei to make another, so they go out walking together staring at girls (and a cross-dressing man) until they end up being arrested for going into the Public Bath's Women section.