Students will engage in an online quiz competition for 5 days on tobacco-related topics. Each day, students could answer a maximum of 10 questions.

Students will engage in an online quiz competition for 5 days on healthy eating-related topics. Each day, students could answer a maximum of 10 questions.

Students in the intervention and control arm will participate in an online quiz game on tobacco or diet related topics respectively. Each day, students could answer a maximum of 10 questions randomly selected from the respective pool of questions by the platform. The students logged onto the online platform and played the quiz competition on weekdays for 5 days.

Students reported their opinions towards 5 statements in pre and post tests: [Pre-test] 1) Smoking causes wrinkles to form on the skin; 2) Third-hand smoke can remain on surfaces for weeks; 3) E-cigarettes can help with smoking cessation; 4) Tobacco companies aim to prevent teenagers from smoking; 5) The Hong Kong government has already banned the sale of e-cigarettes.

[Post-test]

1) Smoking can cause hair loss; 2) Opening windows or turning on a fan can eliminate the effects of thirdhand smoke; 3) E-cigarettes avoid the problem of secondhand smoke; 4) Tobacco companies hire influencers to promote tobacco products; 5) The Hong Kong government has already banned the sale of heated tobacco products.

Students' knowledge will be assessed using a 5-point Likert scale, with each item rated on a 5-point Likert scale ranging from 1 (Strongly disagree) to 5 (Strongly agree). Item scores will be recoded as appropriate, so that higher scores consistently reflect better knowledge.

Students will report their opinions towards 5 statements: 1) Smoking is a selfish behavior; 2) I hope to avoid exposure to secondhand smoke; 3) Vaping is a trend.; 4) Tobacco companies use different methods to attract people to smoke; 5) The government should prohibit people from smoking while walking.

Students' attitudes will be assessed using a 5-point Likert scale, with each item rated on a 5-point Likert scale ranging from 1 (Strongly disagree) to 5 (Strongly agree). Item scores will be recoded as appropriate, so that higher scores consistently reflect more negative attitudes.

Students were asked: If one of your good friends offers you a traditional cigarette, e-cigarette or heated tobacco product, ask you to have a few puffs, and no one will know about this as no more smoking in the future. Will you use them?

Students' intention will be assessed using a 5-point Likert scale, with each item rated on a 5-point Likert scale ranging from 1 (Definitely no) to 5 (Definitely yes). Lower scores indicate a lower intention to use ASPs.

Students were asked: In the past 7 days, how many days (0-7) 1) was someone smoking next to you when you were at home? 2) do you smell secondhand smoke drifting in from outside when you were at home? 3) was someone smoking next to you when you were outside home? 4) you can smell cigarette smoke on objects or people when you are at home and no one is smoking, and you don't see any smoke?

Each item was rated on an 8-point scale ranging from 0 (0 days) to 7 (7 days), with higher scores indicating more frequent exposure to secondhand smoke (i.e., a worse outcome).