Word Choice – ‘My Favourite Food Is’ or ‘Are’

When talking about a particular food or meal, eat and have can function interchangeably most of the time. Of the two, have is the more versatile and generic word:

Let's start with your last example:

I'm eating pizza now. Let me call you back – I don't want my pizza getting cold.
I'm having pizza now. Let me call you back – I don't want my pizza getting cold.

I see no real difference in those two statements. I think I'd be more likely to use the first, but the second wouldn't jar my native ear.

Then your breakfast example:

I eat breakfast every day at 8 o'clock.
I have breakfast every day at 8 o'clock.

Once again, either one of those is okay, although the second sounds a little bit more formal for some reason. In its seventh definition for have, Macmillan mentions:

have (verb) [TRANSITIVE] [NEVER PASSIVE] to eat or drink something. This word is often used in polite offers and requests

• Can I have another piece of that delicious cake?
• Let me buy you a drink. What’ll you have?
• Why don’t you stay and have lunch with us?

I’ll have (=used for requesting food or drink in a restaurant): I’ll have the roast beef, please.

There are a few places where the two words aren't interchangeable. The end of that definition gives one example; if I was ordering at a restaurant, I wouldn't say, "I'll eat the roast beef, please." That might be true, if that's what I'm ordering – but it's simply not idiomatic to say it that way.

Another clue is that have is always transitive. So, it's perfectly fine to say:

I'm starving – let's eat!

but you wouldn't be able to say:

I'm starving – let's have!

Here's one more odd case:

I'm hungry; let's have at that hamburger place.
I'm hungry; let's eat at that hamburger place.

In this case, we can't use have to mean eat, because we're not using the word transitively. We can fix that by saying:

I'm hungry; let's have hamburgers at that place.

However, the first is not necessarily grammatically incorrect, because we could be using the phrasal verb have at. NOTE: This would be a very informal usage of have at, but I give it a mention because it shows how complex and flexible English can be, especially when dealing with informal expressions and eating food. When I was in college, one of my roommates might have said:

I'm hungry; I think I hear hamburgers calling my name!

Your assertions in the first part of your question are correct - we use the word 'brain' both to describe the organ inside our heads, and as a synonym for intelligence. So to quote your examples:

• Use your brain.
• He killed people and ate their brains.

Both refer to the physical organ, in singular and plural form, respectively.

• "Beauty with brains"

In this case, 'brains' is a synonym for intelligence, so again you are correct.

Now,here is the problem that encountered : "We need to put our brain to work to make sure it stays fit for long."

Should not this be brains and not brain ? Because we are talking about our brain that is the brains of more than one people.

Ignoring the oddly worded ending to the sentence, I would argue that the sentence is not gramatically correct. In the first half of the sentence, the personal pronoun 'we' and possessive determiner 'our' imply that we are talking about more than one person, but in the second half of the sentence only one brain is being referred to, as evidenced by the singular 'it', in 'to make sure it stays fit for long.' So it would probably be best to pick a side...

Either

• "You need to put your brain to work to make sure it stays fit for a long time."

or

• "We need to put our brains to work to make sure they stay fit for a long time."

would be good starting points.