Triangle Congruence 1

Interact with this applet below for a few minutes, then answer the questions that follow. As you do, feel free to move the BIG WHITE POINTS anywhere you'd like on the screen!
The labels indicate that: AB is congruent to FG AC is congruent to FH Angle Alpha is congruent to Angle Beta

Q1:

Why do the congruences listed above force BC to be congruent to GH?