An idiom is a fixed phrase whose meaning you can't work out from the words. "A piece of cake" has nothing to do with cake — it means something is very easy. Hit the nail on the head means to say exactly the right thing about a problem. Both idioms are informal/spoken — perfect for everyday talk, but keep them out of formal essays and reports. Learn the whole phrase and its meaning together.
Idioms don't mean their words
a piece of cake /ə piːs əv keɪk/ — means very easy: The warm-up was a piece of cake for Mei.
hit the nail on the head /hɪt ðə neɪl ɑn ðə hed/ — means say exactly the right thing: Amina hit the nail on the head when she explained the problem.
Not ""A piece of cake" = a dessert you eat" — say ""A piece of cake" = something very easy". Idioms carry a hidden meaning. The literal words are not the point.
Try it: Mei finished the exercise in seconds. She said it was ___. (a piece of cake / under the weather / black and white) Answer: a piece of cake — "A piece of cake" means very easy, exactly Mei's experience. "Under the weather" means feeling ill, and "black and white" means clear-cut, neither of which fits "very easy".
Idioms for how you feel and where you are
Some idioms describe a feeling or a situation. Under the weather means slightly ill; in the same boat means in the same difficult situation as others. Both are informal/spoken English — you'd say them to a friend or colleague, not write them in a formal report.
under the weather /ˈʌndər ðə ˈweðər/ — means feeling slightly ill: Sofia felt under the weather and stayed home.
in the same boat /ɪn ðə seɪm boʊt/ — means in the same difficult situation: Everyone missed the bus, so they were in the same boat.
Not ""Under the weather" = talking about the rain or cold outside" — say ""Under the weather" = feeling slightly ill". The idiom is about your health, not the actual weather.
Try it: Sofia woke up feeling slightly ill and stayed home. She was ___. (under the weather / a piece of cake / give and take) Answer: under the weather — "Under the weather" means feeling slightly unwell, exactly why Sofia stayed home. "A piece of cake" means easy and "give and take" means compromise, neither about being ill.
Idioms for work and decisions
These idioms come up in work and study. Call it a day means to stop working because enough is done; pros and cons means the advantages and disadvantages of a choice. Call it a day is informal/spoken, so save it for conversation; pros and cons is more neutral and is fine in writing too.
call it a day /kɔːl ɪt ə deɪ/ — means stop for now: After three hours, the team called it a day.
pros and cons /proʊz ənd kɑnz/ — means advantages and disadvantages: The class listed the pros and cons of online learning.
Not ""Call it a day" = name a particular day or date" — say ""Call it a day" = stop working now because you've done enough". Call it a day is about stopping, not naming a day.
Try it: Before choosing, the class listed the good points and bad points of each option, the ___. (pros and cons / give and take / piece of cake) Answer: pros and cons — "Pros and cons" means the advantages and disadvantages, the good and bad points. "Give and take" is about compromise, and "a piece of cake" means easy, neither of which is weighing options.