While learning torsion i came across formula $$\tau = \frac{\mathbf{r}'\mathbf{r}''\mathbf{r}'''} {\mathbf{r}''\cdot\mathbf{r}''} = \frac{\dot{\mathbf{r}}\ddot{\mathbf{r}}\dddot{\mathbf{r}}}{(\dot{\mathbf{r}}\times\ddot{\mathbf{r}})(\dot{\mathbf{r}}\times\ddot{\mathbf{r}})}.$$
I can't understand how to step from $\frac{d}{ds}$ to $\frac{d}{dt}.$
I tried to unfold derivatives, but it takes so much place what i can't even make into such space that i can see all at once, and anyway i can't bring it to the end.
I understand that triple scalar product is area of parallelogram, so may be it can be explained without so big formulas. Thanks.