Partition of a set, definition not clear

Question

From wikipedia:

Equivalently, a set P is a partition of X if, and only if, it does not contain the empty set and:

1. The union of the elements of P is equal to X. (The elements of P are said to cover X.)
2. The intersection of any two distinct elements of P is empty. (We say the elements of P are pairwise disjoint.)

I clearly understand that the intersection between partition is empty (point 2), but how can the union of a partition can be the all elements in the set?

If it is a partition, shouldnt they be just a part?

I imagine a set divided in 3 and the elements in the first part are not all the elements of the second part.

How do you explain this?

Answer 1

The idea of a partition is that you take a whole (the set $X$) and you divide it to parts.

Now if I cut off an apple into slices (and one core) I have several pairwise disjoint parts of the apple, but if I reassemble the parts I get a whole apple again.

Similarly we require this from a partition of a set. We want that the union of all the parts give us the entire set we partitioned.

Answer 2

The union of all parts gives you the whole set. So if you partition a set $X$ in three parts $P_1$, $P_2$, $P_3$, then $P_1\cup P_2\cup P_3=X$.

Answer 3

The examples will help. Examples of partitions of $ \{1,2,3\} $ are \begin{equation} \{1\}, \{2\}, \{3\} \end{equation} \begin{equation} \{1,2\},\{3\} \end{equation} \begin{equation} \{1\},\{2,3\} \end{equation} \begin{equation} \{1,2,3\} \end{equation} \begin{equation} \{2\},\{1,3\} \end{equation}