Theory:

II
“Please give me a ride on your back!”
Said the Duck to the Kangaroo.
“I would sit quite still, and say nothing but ‘Quack’,
The whole of the long day through!
And we’d go to the Dee, and the Jelly Bo Lee,
Over the land, and over the sea;
Please take me a ride! O do!”
Said the Duck to the Kangaroo.
                
III
Said the Kangaroo to the Duck,
“This requires some little reflection;
Perhaps on the whole it might bring me luck,
And there seems but one objection,
Which is, if you’ll let me speak so bold,
Your feet are unpleasantly wet and cold,
And would probably give me the roo-
Matiz!” said the Kangaroo.
Explanation:
 
The duck is quite fascinated by the lifestyle that the kangaroo leads, as it can jump around and very easily explore new places. When one stays stagnated in a place, it stunts their growth and development. The duck is aware of this fact, yet it does not know a way out. It can hardly move from the pond because of its physical features. It can only trot and flap its wings to a certain level. But the desire to visit places and wander around does not cease to exist. The duck, therefore, makes a request to the kangaroo to give a ride on its back.
 
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Duck flapping its wings
 
When asking a favour, one has to be polite in order for the request to be considered. The duck promises that it would cling onto the kangaroo's back and would stay there silent and not disturb it in any way. It also says that it might occasionally quack as it might get excited by the new pursuit. The duck also says that they can go to Dee, and Jelly Bo Lee, which are only imaginary places used to create an effect. It is also made in Australian slang since kangaroos are from Australia. The duck ensures that it wants to hop over the land and sea and visit all the places.
 
The kangaroo is a very practical thinker and therefore says that it needs time to think and ponder over the request. It has to analyse from all angles based on side effects, drawbacks and advantages etc. In many cultures, a single duck is supposed to bring luck and is a symbol of prosperity. Some believe that a group of seven ducks symbolise wealth and fertility. Although this is an advantage, carrying the duck around can have its own drawbacks. But the kangaroo is very polite and does not wish to offend the duck with mean comments. It is always unpleasant to comment on someone's weight, insecurities, flaws etc., and the kangaroo makes sure that it asks for permission to make a bold comment. It then goes ahead with the remark that the duck, being in the water all the time, has cold and unpleasant feet. When it rides on the kangaroo's back, the feet stick to the body, and extreme exposure to the wet and chill feet may bring in a lot of diseases such as roomatiz. Rheumatism or Roomatiz is a bone disorder caused especially by cold and wet exposure.
 
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Roomatiz is a bone disease
 
Meaning of difficult words:
 
S.No
Words
Meaning
1
QuackThe sound made by a duck
2
ReflectionSerious thoughts or consideration
3
ObjectionDenying something
4
RoomatizA disease related to bone disorder
5
UnpleasantA very state
Reference:
National Council of Educational Research and Training (2006). Beehive. The Duck and the Kangaroo  - Edward Lear (pp.94-96). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.