Will the speed of an electron convert to mass when it is near light speed?

How come speed be converted to mass?

In the "mass energy equivalence" principle by Einstein, mass and energy are equal entities. That is, you could express "mass" in terms of "energy" and vice versa (i.e. express "kg" in terms of "joule" and vice versa).

You could never express "speed" in terms of "mass".
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