Why is signed by taking your dominant hand and making the ASL sign for the letter 'Y' at the side of your head. A variant of this sign is to wiggle your middle finger up and down with the rest of your fingers spread out, palm facing the side of your head. A third alternative is still to make the letter 'Y', but after pulling your hand a bit further down your dominant side.
Whichever sign you use, make sure you furrow your brow to signal to your toddler that you are asking a question.
Use the why flash card when you are ready to unleash the monster. Your toddler's curiosity will know no bounds as they grow in wonder at the world around them!
Toddlers ask about 437 questions per day. Most begin with the word why. Good luck!