You're having a conversation about something with your boyfriend, and he interrupts to ask what's for dinner. You're annoyed that he always talks about food. You say:
Your mind's always on food!
Use this expression to talk about a topic that someone thinks about a lot. Aside from food, here are some of the most common topics that people's minds are "always on":
His mind is always on work.
Your mind is always on sex.
Her mind was always on other things.
Most of the time, when you say that someone's mind is "always on" something, it's a complaint. You want to say that they think about that topic too much.
You can also use the phrase "get your mind off of ___":
It's nice to take some time off and get your mind off of work for a few days.
The speaker in the example says "Your mind's..." This means "Your mind is..."
"Is" can be contracted to "'s" on the end of a noun:
The refrigerator's broken.
Patty's nineteen now.
But the same form is also used to show that something belongs to a person or thing:
The refrigerator's handle is broken.
Patty's brother goes to my son's school.
English speakers have to figure out which way "'s" is being used based on the situation.
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