The cup of black tea was now bought,
and so the white can it was sought,
some crystals were poured,
the drinker then floored,
because it was salt that she got.
This poem is a limerick and is intended to be humorous. The humor comes from the mistake of pouring salt instead of sugar into tea and then tasting it.
The first line of the poem introduces the object for the mistake. A variety of foods and drinks could have been used. Depending on what was chosen, the mistake could have been pouring sugar instead of salt. A cup was chosen to imply hot tea instead of cold tea. This was to help create a visual image in the mind of the reader.
The second line mentions a white can. The color of the can is meant to hint at the contents of the can, by implying that the color of the contents is associated with the color of the can, without stating what the contents are. A can was chosen because it is ambiguous in terms of what it contains. If, for example, a bowl was chosen instead of can, it may have leaned too much toward sugar.
The third line goes with the second line in describing the substance in the can without stating what it is. Crystals can imply either salt or sugar. The substance was poured instead of spooned or sprinkled to continue the ambiguity of what it was.
One aspect of this poem to note at this moment is the idea of an intuitive feeling. The intent of the first three lines is for the reader to intuitively feel that sugar is being poured into tea. Hopefully this scene will be common to the reader. If this intuitive feeling does not happen it will decrease the humorous effect of the punchline. Without the intuitive feeling the reader will have to draw a connection to understand that the first three lines were meant to imply sugar, rather than simply experiencing humor with the punchline. The idea of an intuitive feeling can be important in many poems.
The fourth line introduces a surprise both to the reader of the poem and to the person in the poem. The line also introduces a mystery to the reader of why the drinker was floored.
The fifth line is the punchline of the joke and solves the mystery of the fourth line.
Another aspect of the poem is that although it is about sugar and salt, sugar is not actually mentioned in the poem. Sugar is hinted at by the black tea, the white can and the crystals, but it is never mentioned.