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Take psyllium husk (a tablespoon or two should suffice), cornstarch (again, a couple of spoonfuls), and water (you'll have to eyeball it, maybe start with half a cup). Add food colouring if you wish. Cook them all out in a pot, and you should get a slimy, gelatinous mixture that approximates the slime that contains borax / contact lens solution and glue. The difference is, this one is completely edible. It won't taste good, but you dont need to panic about more than a giant dump if your rugrat decides to sample it.

Needless to say, let it cool before handing it over for play.

Most basically, it's fibre. People take it to help sort out constipation. It is commonly used in gluten-free breads to help with binding the ingredients together (so don't eat too much of those breads in one sitting, otherwise you'll be running to the toilet). You can get it in health food stores, either in capsule format, or loose in a bag. I've also been told you can get it in South Asian grocery stores - it'll be cheaper that way.

I've made pure psyllium husk slime in the past - just cooking out psyllium husk and water. It was jelly-like, but not particularly slimy. So when the M&Ms were badgering me to make slime, I thought to try adding another thickener to it. Corn starch makes a wonderful goop, so I decided to give it a go. They both remember very well how fun goop is to play with, so they were really keen to try it out. The results speak for themselves...