Make any expression interactive.

Start with an expression

Here is an expression that makes a 100-point a in Times font:

Click for copyable input

Make the expression interactive with Manipulate

Wrap the expression with Manipulate, replace the fixed size 100 with a size variable and specify that size should range from 1 to 200 with an initial value of 100. Drag the slider to change the size of the character:

Add a control

Replace "Times" with the variable family that can have the values "Times", "Courier" and "Helvetica". Click a font family to change the font:

Add another control

Replace a with the variable char that can range from a to z. Choose characters from the drop-down menu:

Change a control type

Manipulate chooses control types automatically, depending on the type of variable being controlled. You can override its choice by specifying a control type explicitly. Change the character menu to a slider: