The mind if assumed to be a vessel, shall soon be filled to its brim. Does that mean you can not fill it anymore? You will have to empty it in order to refill it? I don't think so. Why? Because the mind is not a vessel, it is not something that can be quantified by volume. You can't define a capacity, just like how you can't define the amount of love you have for something.

Know something else that can't be defined? FIRE.
What class of matter does it belong to? Does it occupy any space, volume, does it have any mass?

You put some entity into it, there are only three possibilities: The entity might be subsumed by the fire; The fire might blaze further; The fire might die out or extinguish.

The mind, is like fire. All your thoughts are the entities you can kindle it with. Your memories, desires, dreams, fear, everything. 

A successful person might argue that he used his ambitions as a fuel to feed his fire-like mind, that drove him with the energy to achieve something big. A depressed person might acknowledge some maleficent incidents that have prompted him to extinguish the fire of his mind. Your quotidian thoughts like'What to eat?', or 'Where to go?' might be considered the menial entities that get subsumed in the fire. An angry or unstable mind might have gotten incendiary due to one or more reasons (thoughts).

To think of it, we do say, "An idea sparked in my mind!"
What does that mean? A sudden thought, revolutionary or not, that has stirred the fire inside your mind to do something. Right?

When we say, Food for thought I am most certain we are referring to the entities and classifying them into one of the three.

You can never limit your mind to your thoughts. The conscious, sub-conscious, whatever. Mind can not be quantified, nor can its boundary be assumed to exist.
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