To trigger an *object* glyph, a creature must "open" the object: 

> This powerful inscription harms those who enter, pass, or open the warded area or object.... 

But also, the caster can "set all of the conditions": 

> *You set all of the conditions of the ward*. **Typically,** any creature entering the warded area or opening the warded object without speaking a password (which you set when casting the spell) is subject to the magic it stores. 

So if a caster wanted to store a number of different effects in a small container, are their options limited to small, openable objects? Or does the "set all the conditions" clause allow you to *glyph* pebbles that pop when they hit the ground? 

Extra Credit: if the small object must be openable, could you cast *glyph of warding* on an egg?