I frequently change the water in my fish tank (rather, I get S to do it while I watch and give instructions... 'Careful of that fish!' 'Don't touch that!' 'You've missed a bit on the left.').
Why, you ask?
Because I've seen one too many episodes of Ray Mears (thank you, S) and I read 'The Life of Pi' (remember - http://livyspinkpants.blogspot.com/2009/03/my-favourite-animal-is-tiger.html?), and I have daymares (like nightmares, but in daydream form) of needing to drink the water in the tank.
You know, like if there was a massive flood or tornado that somehow barricaded all the doors to my flat, plus crushed the pipes underground so that there was no running water.
In that situation I'd have to drink fish-water, and I need to be prepared for that.
Party Make Up
10 months ago