they're plastic strings ("rexlace") basically, called scoubidous. this time round all the 14 & 15 year olds are doing them. you can plait & braid them into various things, like friendship bracelets and keyrings. http://lartdesscoubidous.nuxit.net/ is a site that shows you some pictures & explains it (in french, sorry). about 10 years ago they were in, but it was 7 & 8 year olds who dominated the craze. my mum remembers them from about 35 years ago, too...
me too!! I had scoobidoos when your mum did. I am 51 now and had them in primary school so that makes it nearer 45 years than 35. Sorry, Magicdice's Mum!!) You can also do scoobidoo with ordinary string or even thick knitting wool and there is a related knot used in macrame called flat sennit or flat bosun, the round version is called rolling sennit. If you kids want to show off, google on "macrame" there are loads of websites with different fancy knots and they are really easy!!
to magic dice i chose this category because i did a search thingy for scoobies on this website and wrote crafts but it said for me to enter my question into this bit!!