Toppings
Sometimes, complementary toppings to the core liquid may be found floating inside of the tail. Examples of this may include marshmallows in hot chocolate, flowers in a floral drink, boba pearls in boba tea, etc. Though not present in all cores, in those that they do exist in they are a non-harmful and natural part of rattleshake physiology.
Topping Sickness
It is crucial to a rattleshake's health that there not be more solids than liquids in the tail, as it will impede circulation of magic in the body and can lead to extreme unwellness or even death. This is an uncommon issue though, luckily. The body naturally breaks down and absorbs excess toppings, converting them to energy instead of keeping them around (assuming the consumable are edible and not, say, plastic).
Absorption
Over time toppings may degrade and get absorbed into the shake's system, such as marshmallows melting or flowers wilting. If this occurs, they may be replenished by the consumption of fresh toppings, which will make their way to the tail.
Compatibility
Only toppings that are naturally compatible will make their way to the tail. If a topping is not compatible, the body will break it down right away in an effort to keep it from contaminating the precious core liquid. This is not ideal, and eating the wrong sort of thing can make a rattle quite queasy (see Diet).