How to I make my carne de lengua more tender?

Question

My favorite taco meat is lengua. Every time I visit the taquería, the lengua is very soft and tender. When I make it, it's tough. How do they do it?

Answer 1

You  can cook it in a crock-pot overnight on a low setting for at least 12 hours. By the time it's done, it will be very tender.  

Answer 2

To boil it for over 5 hours in a mix of vinegar, celery, cilantro and carrots will turn more tender.  

Answer 3

Boil it until a carving fork pierces it easily.  

Answer 4

steam cooking it makes it more tender