In every movement of the universe, the scientific knowledge is inherent. A person walking or swimming or getting up from a chair is under the realm of science. Thus, every event that occurs in our 'daily life' can be explained on the basis of concepts of science.

Force causes 'motion' !Every object in the universe will be either at 'rest' or in 'motion'. If an object at rest is to be set into motion or the object in motion is to brought to rest 'force' is to be applied on it. Further, this force should be an 'external' one. One cannot stand on a football and kick it! A bus that failed to move has to be pushed from outside. It cannot be moved by the passengers inside by pushing the seats in front of them.

According to the famous scientist Sir Isaac Newton, whenever we apply a force on a body, the body at the same time applies an equal and opposite force on us. When a player kicks a football with certain force (called 'action'), the football in turn at the same time kicks the player (!) with an equal and opposite force (called 'reaction').

'Walk' starts like thisIf a person standing still likes to walk, he has to kick the earth with certain force. This is 'action'. At the same time, the earth exerts an equal and opposite force on his feet. This is 'reaction'. Because of this reaction the person moves forward.

Person 'sitting' in a chair 'getting up'Suppose a person sitting in a chair wants to get up, he has to apply some force on the chair. Without exerting force on the chair and his feet not touching the floor, it is not possible for him to get up. Because, if that person cannot apply force on the chair, the chair will not apply 'reaction' on him. Without kicking the floor with his feet even if he exerts force on the chair he cannot get up. Further, while exerting force on the floor, he has to bend a little and bend his legs at the knees. Otherwise, the reaction which comes into play by kicking the floor will be completely balanced by the 'weight' of his body. By holding the hands of the chair, if he bends a little and kicks the floor by bending his legs, a part (component) of the reaction given by the chair and floor balances his body weight and the other part will be helping him to get up from the chair. That is why a person walking also cannot walk without bending his knees.

Why is it difficult to 'walk' on sand?It is easy to walk on a hard ground than on sand. Why?
Sandy land will have a number of layers one over the other. A person walking on the sand will exert a force with his feet to start with will be experienced by the top most layer of the sand. That sand layer in turn will transfer that force to the layer below it and soon. Thus, the force exerted by the feet of the person will take some time (however small it is) to reach the lowest sand layer. Moreover, certain amount of this force will be absorbed by the sand layers. Because of this, in force in the form of reaction exerted by the sand will be less than the force applied by his feet on the sand. Further, this reaction will take certain time to come into play, where as the reaction in case of hard ground comes into play, as soon as the force is applied by the feet of the person.

This is also the reason why a person hurts more when he falls from a height on a cement (hard) floor than on sand. Now one can understand easily why the sports pitches in case of long jump and high jump are filled with sand.

This is how a person swims!If a person wants to swim in water, he or she cannot simply lie flat on the surface of water. One has to lie flat facing the water and apply certain force on the water by pulling certain amount of water towards him by his hands and at the same time moving his legs. This is the 'action'. At the same time the water will exert an equal and opposite force on him (reaction) because of which he moves forward without drowning.