this is how i handle this situation... stop, turn your thoughts around about 90 degrees and start again. in other words... they don't understand it and you explaining it the same way over again doesn't help them. try explaining it in another way... eventually you'll find a 'way' that works for them.

if possible... use something visual to assist them. as a homeschooling mom, that's what i do when my kids just don't get it. bottom line: smarties always assist a situation. don't know why... but they do and they are a very useful teaching tool! *who knew?*