Giant Eggs in Netherlands

So I came across this today on Toxel.com and thought this would be great to share. This breakfast inspired environmental installation by Dutch artist, Henk Hofstra, can be found in Leeuwarden, Netherlands. The piece is made up of several 100 ft wide sunny side up eggs and will be on display for the next 6 months.

I especially love how Hofstra created the three-dimensional yolks (seen below). I would love to know what type of medium he used. Does the yolk inflate, and is it made of a similar fabric, such as the inflatable snowmen found in so many people's yards during the Winter holidays? Or, is it hard and made of molded plastic, plaster, or metal? Maybe it is one giant yoga ball (and I mean giant) cut in half and secured to the pavement by caulk? Whatever the medium used, what a happy and unexpected art installation.