19. (3 points) As shown in the figure, an isosceles triangle $A B C$ is folded along $E F$, with vertex $A$ coinciding with the midpoint $D$ of the base, if the perimeter of $\triangle$ $A B C$ is 16 cm, and the perimeter of quadrilateral $B C E F$ is 10 cm, then $B C=$ $\qquad$ cm.
【Solution】Solution: The perimeter of $\triangle A B C$ is 16 cm, so the perimeter of $\triangle A E F$ is: $16 \div 2=8$ (cm), the sum of the perimeters of $\triangle A E F$ and quadrilateral $B C E F$ is: $8+10=18$ (cm), therefore $B C=18-16=2$ (cm), answer: $B C=2$ cm. Hence the answer is: 2 .