Thin-sliced boneless top round, very lean.
Marinated - heck, SOAKED - 5 hours in regular Coca-Cola. Fried up (very briefly) one slice to test at lunch - a little (but VERY little) less tough than the one I cooked at the beginning of the day.
Added a good splash of lemon juice and gave it another 5 hours, BARELY cooked in wok and served - result: exactly the same, fairly chewy.
Long ago I tried the method with the baking soda, same result.
Where is the method for getting the beef in the beef-and-broccoli I bought at takeout last week? It was practically glutinous (easily as tender as, for instance, a grilled sea scallop).