 So look at an example here with phenolphthalein, and I'd like to thank Rhysluis for this animation. So as we play the animation, we're changing the pH. We can see we've got hydronium ions there, water in the low pHs turned into hydronium ions because of the high D9s. As the pH increases, we turn over to hydroxide ions being present in the water. As that happens, we get a change in some of the bonds on the phenolphthalein molecule. When that happens, we get a bit of a change in shape, a slight change in shape there. That change in shape leads to a change in the way the light reflects off of the phenolphthalein molecule. In this case, it reflects wavelengths of light to make up pink in our brain. When we reverse the pH, we go back to being an acid, the molecule changes shape back, and we get back to the colis situation we were in. So the phenolphthalein, when it's in acidic up to about a pH of 8 or 9, it's colis. So here we can see over here, we've got a picture of the acid form where it's colis, and over here we can see the basic form where it's pink. That subtle change in shape leads to the change in the way the light reflects off of it, and that changes its color. So when phenolphthalein is colis, it's in acidic solution, when it's pink, it's in a strong basic solution.