Why are transition metals called by this name?

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The elements in the periodic table are often divided into four categories. They are the main group elements, the transition metals, lanthanides and actinides. The transition metals are the metallic elements that serve as a bridge or transition between the two sides of the table.
Transition metals look like metals, they are malleable and ductile, and they conduct heat and electricity. However, they are lee like each other than other groups of metals. For example, tungsten is a transition metal and it has the highest melting point of any metal. On the other hand, mercury is also a transition metal and it is liquid at room temperature.