We're well into July now so this is just a quick challenge that won't even apply to everybody. But if, like me, you have a shower unit over your bath (rather than in a separate shower cubicle, or even no shower at all) then I challenge you to put the plug in next time you take a shower.
You'll end up standing in a few inches of water but never mind, I only want you to do it once. The reason is to see how much less water you use in a shower than in a bath. Unless you take very long showers, you won't use nearly as much water as if you filled the tub for a hot bath.
Most people know that showers use less water than baths. But I think it's really helpful to see with your own eyes just how much difference there is. Direct personal experience is a lot more likely to make you want to change your behaviour than any efficiency campaign or general good intentions.
Once you've finished taking your shower, consider what you're going to do with the water. Will you just pull out the plug and let it go down the drain? Or can you reuse some of it, perhaps to water plants? If it's still quite warm you could mix a little soap powder into it and drop some laundry in there. I wash my duvets in the bath once or twice a year because they're too big to go in the machine. It's better to reuse bath or shower water for a job like that than to fill up the bath just for a duvet. Or if you are clever at plumbing, could you somehow divert the water into your washing machine, or an irrigation system for your garden, or even your own reed bed?
But that's not the main focus of the challenge. The challenge is just to put the plug in and see how much water you save by taking a shower rather than a bath. Don't forget to vote in the poll in the right-hand sidebar when you've done it.