example of different logic gates, NOT, AND, OR, XOR and NOR. To change the inputs on the left click them or hit the green flag to randomly set the inputs.
Compare the logic gates used in Minecraft and Redstone (http://minecraft.gamepedia.com/Tutorials/Basic_Logic_Gates) with the expressions possible here in scratch using boolean expressions. Programming languages typically include boolean typed variables and this is good practice to see if you can build more complex logic gates by remixing this project.