Ivory soap has a marketing campaign that trumpets how their product floats in water. To make themselves seem more modern, they could show people what Ivory does in the microwave. Given what happens, it's unlikely they will, though.
Sometimes it seems justified to wonder if anyone uses the microwave for food anymore. There are so many other wonderful uses for it, from melting down toys to getting water to explode. It seems almost a waste to use it to feed people. One of the many, many things that does cool stuff when shoved in a microwave is soap - in particular, Ivory soap.
While most soaps sink sullenly to the bottom of the bathtub, the sink, or the puddle in the prison shower, Ivory does something else. It floats on water, bobbing up and down on the ripples created by the standard prison-issue rubber duckie. Throughout Ivory's history there have been ads suggesting that Ivory soap floats on water because it's light and because it's pure. That's crap. Gold can be pure, but it certainly won't float on water. Pigeon feathers are light, but no one wants to scrub their unmentionables with those.
Ivory soap floats on water because it's less dense than water. Solid soap heads right for the bottom of the pool, meaning it's denser than water, so it's not any special formula that makes Ivory float. Instead, it's the manufacturing process. Little air bubbles are pumped through the soap, so the total volume of the soap is part soap, part air. This makes it less dense than water, and allows it to float.
It also allows it to do something kind of interesting in the microwave. The microwaves heats up water in food. The bubbles of the soap have some moisture in them, and the soap itself contains some water. As that water heats up, it causes the tiny bubbles in Ivory to expand. As they expand, they push the soap out of shape, making it fizz up to many times its original size. When it comes out of the microwave, after about a minute, it feels like leftover soap bubbles that have dried in the tub. It's still usable as soap, but it won't be in a compact, convenient package anymore. And it does look like an albino blob.