Kids seem to be great at pushing boundaries, some more than others, depending on their character. To a tired and stressed parent, it can seem like their child is only out to torment them.
Nothing is less true...
Indeed, children push the boundaries and test their limits, but that is only their job. We shouldn't take this personal.
Kids' sole purpose in the early years is to grow and learn, and they learn by discovery. Boundaries and limits is a part of their world, so it is only natural that they test them.
Most children are extremely empirical and will test and test again to verify if they get the same result each and every time. If this is not the case, this only gives ason for retrial.
In keeping a calm composure and not letting this testing tick you off - thus, not taking it personal- in seeing this behavior as what it is: a manifestation of your child's inquisitive nature, you are offering them a strong lesson in human behavior, one that will be the ground for their future connections to other people.