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Welcome to ITT v4 form to science today's lesson is sensory organs and their functions we already looked at sensory organs and some of their functions when we were doing our science in primary school the main sensory organs that we learnt in primary school are very similar to the sensory organs that we learn when we are in secondary school the sensory organs are the eyes the ears the sensory organ of taste which is our tongue the sensory organs of smell which is our nose and a little bit of the tongue as well as we will see later on and also the sensory organs of the skin or the joints or any other part of our body where we might feel something now we really do need our sense organs because what they do is they allow us to detect what changes in our environment there are many stimulus these stimulus are actually the changes in the environment around us if the light becomes dark if a smooth object becomes rough if a dull object becomes shiny all of these are changes that we detect and any change that we detect we call a stimulus a stimulus is due to a change in the environment around us now this is the definition of a stimulus and you must know this definition it does come out in your exam they will say what is a stimulus okay and you need to be able to say it is a change in the environment around us stimuluses can also come from inside our body such as we get too warm or we feel dehydrated which means we don't have enough water or we feel out of breath after running which we don't have enough oxygen in our body but for the purpose of this chapter we will mainly look at stimulus that come from outside our body examples of stimulus sound light pressure and chemicals now why do we need to know these stimulus why do we need to be able to detect them well as scientists we do experiments whenever we do experiments we have to observe what is occurring most of our observations occur using our sensory organs let me give you one or two examples for example this water over here just by touching the glass I will know whether or not it contains cold water or warm water now in an experiment we sometimes need to feel whether a reaction is becoming hotter or becoming colder which we call endothermic or exothermic there are also other things that we need to do in a laboratory that depend on our sensory organs picking up different stimuli for example I'll move this water away sometimes we need to know if something is smooth or if something is rough we also need to know if something is sharp like this and can poke you or if something is rounded and is not so dangerous when it hits you we do experiments in the lab using acids and alkali if something is an acid we need to know that it's an acid here we are using our sense of sight this is a bit of litmus red litmus paper when I put this red litmus paper into this liquid I will know whether the liquid is acid or alkali based on the color change now this is using light stimulus I put it in like so and I notice that...