Smooth function conditions

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A curve defined by $x=f(t)$, $y=g(t)$ is smooth if $f′(x)$ and $g′(x)$ are continuous and not simultaneously zero.

Why do we have the second condition(simultaneously zero)?

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Think of a particle moving along the curve. At any point where $x'(t) = 0$ and $y'(t) = 0$ the velocity of the particle is zero. After coming to a stop it can begin moving again in an entirely different direction. Thus even if $x'$ and $y'$ are continuous, the direction of the tangent vector can change abruptly at any time $t$ that both $x'$ and $y'$ both vanish, and is thus not "smooth".