Pick one of the 5 congruence conditions below and enter your own numbers. Enter values matching the condition and a △DEF exactly the same size and shape as △ABC gets built — press the "Overlap" button and you can watch the two triangles line up perfectly.
Two triangles are congruent when they can overlap perfectly. The key idea behind congruence conditions is that you don't need to measure all six elements (three sides, three angles) — matching just a handful of conditions is enough for the rest to automatically match too. The main ones are SSS (all three sides equal), SAS (two sides and the angle between them equal), and ASA (one side and the two angles at its ends equal).
This same principle is what lets one blueprint produce many identical parts. Without re-measuring every dimension, matching just a few fixed lengths and angles is enough to reproduce a shape that's exactly the same as the original.
Pick a condition, enter your numbers, and build △DEF. Press the "Overlap" button and check for yourself whether the two triangles really do line up.