Nintendo Ambassadors now have 10 free Gameboy Advance games on their 3DSes, which is wonderful. Some of these games look like they were thrown onto a freeway during rush hour in Freight-Truck Town, which is not wonderful. The 3DS' screen is 400x240 pixels, while the classic games are set at GBA standards, 240x160 pixels. When loading a GBA title normally, it will stretch out to fill the 3DS screen and look like the elastic on your 30-year-old "lucky" pair of tightie whities -- but never fear, because you can revert the game to its standard size, as provided by 1UP.
Launch a GBA title on the 3DS and hold down the select or start button until the boot screen disappears and the game begins -- the playable screen will shrink down to standard GBA size, with black bars filling in the extra space around the smaller area. Think of it as a frame for your nostalgia.