Table 1. GuessWhat game elements and treatment focus areas. The parent holds the phone on forehead, variable image prompts appear on screen, and child acts what is displayed ( Fig. 1 ). If parent can guess the prompt correctly, they tilt the phone downward to earn a coin and move to the next prompt. If unable to guess, the parent tilts the phone upward to skip to the next prompt. The game in its current form comes with over 40 different deck choices that can be used alone or mixed into sessions lasting 90 seconds each. Earned coins enable unlocking of new decks and the purchase of new game characters desired by the child. The mobile application is available on Android and iOS; both versions are linked from guesswhat.stanford.edu .

GuessWhat game element	Treatment focus area
Charades: This is a primary game mode. Act out images displayed on screen. Vary the acting challenges and try to score higher with each session.	Eye contact, joint attention, theory of mind, imitation, expressive and receptive communication, nonverbal social awareness, conceptual (abstract) reasoning; flexible thinking.
Selecting and mixing decks: Choose from 40 different decks that feature different animals, sports, occupations, environments, and more.	Self-initiation, independence, comprehension of and response to multiple cues, reinforcement of variable interests/behaviors.
Earning coins: Correct guesses by play partner rewarded with coins that allow purchase and unlocking of new game decks and game characters.	Social motivation
Turn taking mode: Child and parent prompted to control the phone and take turns acting out the challenges displayed on the phone screen.	Turn taking, sharing, social initiations
Video playback: Watch yourself perform the challenge.	Reinforcement learning and course correction
Motion challenges: Sports, gestures, objects, chores decks.	Gross and fine motor skills
Sound challenges: sing-along, pledge allegiance, poetry recitals.	Speech and language
Emotion challenges: emojis, human faces decks, cartoon character faces.	Facial emotion recognition and expression