Creating new variables in Scratch (i.e. explicit variables) can be a conceptual leap for students. To help ease this transition, help students see that they’re frequently using implicit variables such as coordinates, direction, size, volume, instrument, tempo, pen size, and pen color. Help students see that they are using variables when using these implicit variables!
Example: "Try setting or changing the volume variable."
See this and related tips as a Tip Sheet: http://csteachingtips.org/tips-for-teaching-scratch