Synonyms

FRAGILE 

brittle - care - delicate - flimsy - frail - handle - weak - weedy

ROBUST

able-bodied - brawny - cob - flourish - full - hardy - healthy - heavy-duty - robust - rugged - stalwart - stocky - stout - strapping - strong - sturdy - tough - upstanding 

Source: www.wordreference.com 

Freezing Objects

Be believe that ice objects represent very well our two opposite concepts because although the ice can be very strong and resistant to be carved into objects such as this violin, an ice sculpture or even make a shelter, at the same time it may also be easily  shattered into many pieces or it can melt with the rise of temperature, which makes it very fragile.

The egg: robust or fragile?

We have chosen the egg as the object to represent both Fragile and Robust because due to its physical proprerties it can be both. In nature the egg has to be robust enough to protect the baby chick from it suroundings, and the mother has to be able to sit on top of it and warm it without this one breaking. Meanwhile it also has to be fragile enough so that the baby chick with almost no strength can break through it with it weak beak. 

If you analyze the antomy of an egg you will discover that it sides are very weak and can be easily broken (thats why we crack the sides of an egg and not its top) while the top and the bottom are the stongest parts of the shell that can resist enormous amounts of weight and pressure.