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what are cells
what is an organ
what is an organ system
respiratory system
circulatory system
excretory system
skeletal system
what is digestion
where does digestion take place
what is digestive tract
what are the types of digestions
what happens in a mechanical digestion
what happens in a chemical digestion
what are the four types of teeth that humans have
what is an oesophagus
what is peristalsis
what is sphincter
what happens in the stomach
