I saw someone do a demonstration once, they took a box of "iron-fortified" breakfast cereal, dumped it into a bowl, then ran a magnet through it. The magnet picked up some of the dust from the bottom of the bowl, that dust being the tiny iron particles that were added to the cereal to "fortify" it.
I put them in quotes as the word has no objective meaning as applied to a breakfast cereal, it's simply a marketing term. I did not intend to imply that ingested iron particles are not a valid source of iron for human biology.