Contentment is what keeps the cup submerged in water
You should also remain like the bubble in the stream
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From translation of Bang-e-Dara by Dr. M.A.K. Khalil. Web reference is given below.

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Note added at 11 hrs (2008-05-18 02:46:47 GMT)
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I've given the translation five points for confidence since it has been done by a scholar of literature in this field. Here's my interpretation of the verse which I would rate lower on the confidence scale:

A cup filled with water looses aspirations for more, and thus content, remains submerged in water, unable to rise above it. But you should be like the bubbles that water cannot keep submerged, and that always rise above the surface of streams and runlets, attempting to break free.

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Note added at 11 hrs (2008-05-18 02:48:44 GMT)
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Here's the intro to the poem given by the translator:
This long poem, in eight parts, each with a different cadence, deals primarily with the slavery of the Indian sub‑continent and the lack of proper efforts to gain independence. The first part shows frustration and disappointment at the helplessness of ordinary men, who are without the warmth of Khudâ. The second part deals with Man's inability to understand his own reality without Divine guidance. In the third part there is a hint to the poetic genius of ‘All«mah Iqb«l with which he resolves to infuse a new life in the inhabitants of India. It also shows disappointment at the political slavery of India and the state of helplessness as well as lack of struggle and planning for gaining independence. This is followed by an eloquent expression of the resolve of ‘All«mah Iqb«l to break the inaction and inertia of the Indian people. In the next two parts he deals exclusively with the ethical decline of Muslims in general and that of those of India in particular. This is followed by corrective measures in the next two parts. These measures consist of strengthening the struggle for independence and gaining strength for the same from the Love of God and ethnic amity among the different people inhabiting the Indian sub‑continent.


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Note added at 2 days9 hrs (2008-05-20 00:20:53 GMT) Post-grading
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Glad to have helped!