It is truly gratifying to get evidence that Prince William, Kate Middleton, and their three children are remarkably normal. For example, they look a little excited and stressed as they head off on a secret vacation at Heathrow Airport. (Photo. Note Kate's adorable traveling outfit.)

But we are quickly reminded that in most other ways they are very different from us. For example, that they go straight to the ultra-luxurious Windsor Suite at Heathrow Airport. Or that, as members of the farm, they are not really supposed to travel on the same plane. Seriously.

According to centuries-old law, royalty should not travel together. That meant separate planes, separate cars, and at one point, separate carriages. The heirs to the throne could not travel together in case something tragic happened on the road. This is why the Queen and Prince Charles never travel together, and why William and Charles are not supposed to travel together.

So how will this vacation go with William and Kate's three children in tow?According to Express, the Queen can allow the family to fly together, and travel with children has recently been permitted by the Crown Prince.In 2014, Prince William and Kate brought baby George to New Zealand, they had to get special permission. And even if they are granted an exception, there are only a few precious years left when they can even do that: as the third in line to the throne, Prince George will travel on a separate plane from Prince William when he turns 12. That would be an intense conversation to have with a 12-year-old.

It is still unclear where the family is headed, as the trip has not been publicly announced. We will update as further information becomes available.