The reaction of an acid with a base, for example, results in the creation of a salt and water. Some, but not all, reactions can be reversed. See nuclear reaction. An action that results directly from or counteracts another action, especially the change in a body's motion as a result of a force applied to it.
Snakes are cold-blooded creatures, just like amphibians. When a certain animal is cold-blooded, it basically means that they are able to adapt their own internal temperature to that of their environment. When a snake is in a cool environment, their body temperature tends to drop rather quickly.
The common ancestor of birds and crocodiles is much more recent than their common ancestor with mammals. And crocodiles are not warm-blooded. And, AFAIK, there are no known species which have lost the ability to control their body temperature. So, the answer is that we can assume with somewhat hight confidence that the last common ancestor of birds and mammals was not warm-blooded.