Children Learning About Second-hand Smoke: Cluster randomised-controlled trial Breathing in other people's exhaled smoke is called second-hand smoking. Second-hand smoke (SHS) contains harmful chemicals and is a serious health hazard to non-smokers. SHS is estimated to cost more than 800,000 lives per year, worldwide. Children are worst affected; almost a third of deaths from SHS exposure occur in children. A large proportion of the overall burden of diseases due to SHS in children is due to lung diseases such as asthma and chest infections and lung cancer. SHS exposure can also lead to meningitis in children. Children exposed to SHS are more likely to be hospitalised, fall short of their academic potential and take up smoking themselves as compared to those living in smoke-free environments. Recognising SHS as a public health threat, most countries have introduced bans on smoking in enclosed public spaces, which has significantly reduced adults' exposure to SHS. However, for many children, cars and homes remain the most likely places for them to breathe SHS. The only possible way to protect them from SHS is to make cars and homes completely smoke-free.  For the last few years, we have been working with schools, parents and children to develop and test a school-based intervention called, 'Smoke-Free Intervention (SFI). It consists of six teaching lessons delivered by schoolteachers, four fun activities and one educational take home resource. Teaching lessons help to increase pupils' knowledge about the harms caused by breathing second-hand smoke. Fun activities include storytelling, role-playing, quizzes and games. These activities help to motivate children to act and feel confident in talking to adults to persuade them not to smoke inside homes. The take-home resource helps children to show what they have learned in school and to negotiate with their families to "sign-up" to a voluntary contract to make their homes smoke-free. The results of this work show that it is possible to encourage children to discuss with their families ways of restricting smoking inside their homes. Our pilot study also showed that it is possible to recruit and retain schools and children and collect the necessary data for such studies. Inspired by our pilot work, we now propose a large study in Bangladesh and Pakistan where SHS is a major public health problem and a priority for policy makers. Through our work in these countries for several years, we have established collaborations with schools, local communities and policy makers. We wish to examine how effective SFI is, in reducing children's exposure to SHS in homes. We are also interested to see if SFI can improve children's lung health, academic performance and general quality of life, and if it can reduce their health service use. To provide accurate answers to these important questions, we will recruit a total of 66 schools and 2,636 children between 9-12 years of age. We will randomly allocate schools to two arms: arm 1 and arm 2. School teachers in arm 1 will receive training and resources on SFI. School teachers will then deliver SFI in classrooms to the children. Those in arm 2, will not deliver anything about SHS to the children. They will receive SFI to deliver only after the completion of the trial. We will also use objective measurements including testing children saliva for cotinine (a chemical compound derived from nicotine) to assess whether our intervention has reduced their SHS exposure. We will also record their respiratory symptoms such as coughing and wheezing and measure their lung functions and quality of life. We will undertake these measurements as described above using internationally agreed standards. During the study, we will repeat these assessments at regular intervals. Using economics, we will also assess whether SFI is value for money. We will run discussion groups with some children to ask about their experience of negotiating a smoke-free home; and interview some parents and teachers to investigate the likely obstacles to implementing SFI. The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.