Theory:

The poem revolves around a small incident that takes place in the speaker's garden. The speaker seems to be a child when the scenario described in the poem takes place. The poem begins by revealing the main event that led the speaker to write. He sees a snake in the garden. Generally people have their own misconceptions and understandings on reptiles and other creatures. Sometimes they are purely led by fear. They do not try to go beyond the existing beliefs and fathom into reality.
 
As soon as the speaker sees the snake, he runs away in the fear that it might cause any harm. He is led by the knowledge that he has gained from his young age. When a child is born, he does not have any beliefs or knowledge about anything. As he grows and transforms into an adult, the society feeds him its own ideals, thereby restraining him from forming his own opinions from his experiences. The poem clearly depicts this idea with the line, "Some snakes are dangerous, they say". 'They' refer to the society and their forced ideals. But at times, children can be guided in a correct way by sensible adults. In this case, the speaker's mother who corrects his distorted belief in snakes. His mother has a better exposure and with one look at the snake, she confirms that it is a harmless one. She even says that these kind of snakes in fact do good to the society.
 
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Reptiles
 
The speaker sees a garden snake and gets scared. He thinks that all snakes are supposedly dangerous. He does not know to differentiate the good ones from the harmful ones. His mother, having some expertise in that area, advises him that the particular snake was a good one. The speaker and the readers are left to wonder if good and snake is an oxymoron. An oxymoron is when two completely opposite words are put together to make sense. For example, 'pretty ugly' is an oxymoron. But the speaker's mother explains the logic behind referring to the snake as good.
 
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He runs away seeing the snake
 
A food chain is a link where organisms are related by the food they eat. Every organism in the world has a purpose for its existence. Likewise, the garden snake consumes insects and flies as its food. This helps to get rid of pests and insects. If not for the existence of the garden snake, then the garden would be filled with insects, which in turn would spread deadly diseases. The speaker understands the concept after his mother explains it. He therefore does not run away or try to harm it.
 
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Food Chain
 
The garden snake just moves around on the grass in the garden. It does not attack anyone as long as it is not disturbed. It stays in its own zone and serves its purpose. The poet meanwhile stands aside and allows it to pass peacefully. The line stresses on the concept of co-existing peacefully with our eco-system. The poet tells himself that there is no regret in letting the snake live as it is a mere harmless being trying to survive in this world, just like the other creatures.