Example for a <sprite clicked?> block in progress. Each time you only hold your mouse down to the cat, the cat will keep moving and change color unless you move away. But If you move your mouse to the cat while you mouse is still held down, nothing does unless you release and hold down your mouse to the cat again. There is also <Stage clicked> where you have to not click on any sprite, but clicking the background of the project..
This project will look like making a <sprite clicked?> block. See the forum post for more details: http://scratch.mit.edu/discuss/topic/44059 Support This Block to have it work without the workarounds!