There are spells, which can create a bead like object which will explode after
a time. Otiluke’s Freezing Sphere is the one I am particularly interested in (incidentally for the same bard my earlier question about Destructive Wave was for), as it is rather safer to handle than Delayed Blast Fireball…
Relevant snippets from spell description:
A frigid globe of cold energy streaks from your fingertips to a point of your choice within range, where it explodes in a 60-foot-radius sphere.
You can refrain from firing the globe after completing the spell, if you wish. A small globe about the size of a sling stone, cool to the touch, appears in your hand. (…) You can also set the globe down without shattering it. After 1 minute, if the globe hasn’t already shattered, it explodes.
How much cover can block the spell effect?
Example cases to make answering easier:
The spell globe is suddenly placed on the table in front of you by a Mage Hand. It may explode at any time. Clever as you are, you cover it with your brass beer mug and wait 1 minute. Did you just foil a level 6 spell? What if you covered it with your cloak? Or a napkin?
You, the mighty culinary wizard, cast the spell, put the globe into a jar filled with cream and berries, close it with a lid, and give it to a servant to hurriedly take it to the king’s feast. Did you just initiate an assassination, or send out an exquisite ice-cream dessert?