Possibly worth reading a bit about how to check the chain too. How will you know it's knackered (before it snaps or ruins the gearbox bearings)? Here's my brief version, but I'd encourage you to read up a bit, I'm no expert, just a person with a bike. The "stretch" in the chain is due to the links wearing - the holes elongate a tiny bit. This doesn't always happen evenly, and you might find that 99 of the links are fine and one is stretched. This stretched one won't sit properly on either sprocket, and you'll end up with a "tight spot", meaning the (otherwise correctly-tensioned) chain is too tight when in a particular position. You need to do at least one full rotation of the chain, checking tensions every 10cm or so. Only if the tension is acceptable in all chain positions, is the chain good. Hope that helps!