A flight recorder, also known as a black box, purportedly recovered from the crashed Ukrainian airliner, Boeing 737-800, is seen in this still image taken from a video, in Teheran, Iran January 10, 2020.  IRIB VIA WANA/Handout via REUTERS  THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY.  NO USE IRAN. NO USE BBC PERSIAN. NO USE VOA PERSIAN. NO USE MANOTO. NO USE IRAN INTERNATIONAL. NO USE IRAN INTERNATIONAL. NO THIRD PARTY SALES. NOT FOR USE BY REUTERS THIRD PARTY DISTRIBUTORS. IRAN OUT. NO COMMERCIAL OR EDITORIAL SALES IN IRAN.   NO RESALES. NO ARCHIVES  REFILE - CORRECTING YEAR
One of the flight recorders reportedly recovered from the crashed Ukrainian airliner in Tehran. Reuters

What is a black box and how does it survive on impact following crashes?