In this Number Ordering App, participants will be shown three numbers on the iPad screen. In particular, a fixation cross appears in the center of the screen for 500ms, follows by 500ms later by a stimulus (e.g., 2 4 3). Participants will be asked to touch the number that comes first, next and last in sequence as fast as possible. After the practice trials, 24 sequences (ranged from 1 to 9) will be presented to participants. Participants who fail to respond after 7 seconds will be led to the next trial. Experimenter could use the configuration screen to setup number of practice trials, the time limit for each trial, and in-app instruction.