The article reveals the importance of demonstration tasks in physics teaching in secondary schools. Four demonstration of the liquid and gas properties are presented and analyzed. They are: Pressure dependence of gas flow speed; Pressure reduction in air-stream; Wing lift; Liquid surface tension. Reasoning schemes for giving a logical sense to these physics demo tasks are developed. The proposed reasoning schemes reach to activate students' thinking, understanding the demonstrations during the

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... bserved physical phenomena, i.e. help students to: understand the nature of the demo task, determine cause - effect relationships and dependencies, compare conditions and findings, summarize the results, do conclusions. Such using of demonstration tasks is useful to both of teacher and pupil: teacher controls the content of teaching and a learning of pupils, pupils - are focused to self-activities, encouraged to think, analyze, summarize and do conclusions. Key words: physics teaching, demonstration task, liquid and gas properties, reasoning schemes.