Contrary to popular belief, Nintendo DS games are not made when two Game Boy Advance games love each other very much. They're made in factories.
Like this one! Look at those machines go. Sorting, spinning, packing, spinning...fascinating. You really need to watch this 2-3 times, to let the action really wash over you.
UPDATE - For those wondering, seems the cartridges in question is likely Mario vs Donkey Kong 2: March of the Minis.
[via Tiny Cartridge]