0 a round object made of bath salts (= grains of a pleasant-smelling substance that you can add to bath water) that dissolves with a lot of fizzing (= small bubbles and a hissing sound) when you drop it in the water:
Lie down in a comfy bath, put in a bath bomb and watch it fizz and whizz around the tub.
I think I have a glittery bath bomb at home. Sparkly bath for me tonight!
You can make your own effervescing bath bomb.
