EVER wonder why the bottom of a soda can is concave? Why a person won't be swung off the ''Octopus Machine''? How a television works? You will find all the answers at the 26th Joint School Science Exhibition. The one-week exhibition features science projects with the theme ''Ever Wonder Why'' from 30 secondary schools which won the proposal competition held in March. The overall championship went to Hoi Ping Chamber of Commerce Secondary School which submitted a project on soft drinks. The school's display explores the carbon dioxide (CO2) effect of fizzy soft drinks and the shape and design of soft drink cans. Yeung Hon-kuen, the project holder, said his team got the inspiration of using fizzy drinks as their theme after a football game. ''A group of sixth-formers were hot and thirsty after the game. Someone who had a soft drink gave a loud belch, and the others started making fun of him. Then we started wondering why soft drinks did that to us,'' said Hon-keun. More than 20 Forms 4, 5 and 6 students from both the science and arts classes started researching the topic at the beginning of January. Hon-kuen said they started with the children's library, where they found the basic information on soft drinks. They also consulted their physics and chemistry teachers and their sponsor, a soft-drink company, which gave them at least 600 cans of carbonated drinks for experimentation. ''CO2 is added in the manufacturing process under high pressure. As the liquid runs into our stomach, a chemical reaction takes place and the CO2 escapes, which explains the burps,'' Hon-kuen said. Apart from written explanations and colourful models, the team also presented their research in animation on computer, which Hon-kuen said was the most difficult part since they did not know much about software. This was the third year overseas teams were invited to join the exhibition. Representatives from Taiwan, China and the Philippines displayed their science projects. Li Ren-hua, a student of the Korean Secondary School in Changchun, China, invented a machine which cuts Korean pickles 10 times faster than the human hand. The 18-year-old Korean girl said she spent one year to design the device, which is suitable for both home and restaurant use. Tomorrow is the last day of the exhibition, which also features lectures and film shows.