Why do kids act so crazy around their grandparents?

Now I realize that to a child, grandparents are like super-stars. It is a very exciting moment when young children see their grandparents. But seriously, ours go crazy. It often annoys me. They often don't even have any fun because they are in trouble all the time, because they can't sit still or talk at a normal level. My three sweetie-pies get very loud, very active, and start bouncing off the walls, literally. There is this attempt to see who can impress Grandma and Grandpa the most? Who can get them to look at them? Who's art work will they like the best? Who will get the most praise? Who gets the most hugs?

Today was no different. The kids were very cooperatively helping me pick up the living room when grandma and grandpa show up. Immediately they forget all, and start running to the door screaming. (my headache started at that very second) And the chaos began. Within minutes both boys are in a time out for fighting (which the haven't had a time out all day). They were only here for 20 minutes and the loudness, fighting, whining and high level activity, oh, did I mention crying, all together was enough to make even the grand-parents head for the door.

I have talked to my kids about this many times. For years. I don't understand how they can go a whole weekend and just have fun, be normal, get a long, and then at a time you would think would be the most fun time, they can't handle it. Numerous times before and after we go to see grand-parents we talk about what is expected, we talk about proper behavior, etc. But, all is forgotten the minute they see their grand-parents.

Any thoughts, any advice? This is so frustrating to me. It seriously gives me a headache and makes it no fun for me. Who wants to have to put their kids in a time-out when they have special people over to visit.