There are expressions in English that contain the phrases 'in case' and 'in case of'. Though related, these two are used in somewhat different ways. Let's consider 'in case of' first.
In case of fire, everyone should use the staircase.
Call me in case of any emergency.
They're going to complain to the boss in case of the slightest disturbance.
This type of sentence generally tells you what to do or what should follow if something occurs. It's something done in a given situation after that situation appears. What you get from a statement with 'in case', on the other hand, happens to be something else.
Take an umbrella in case it rains.
I'll have some cash with me in case they don't accept credit cards.
Be prepared to give an explanation in case the boss finds out.
She took an extra pen with her, just is case.
In case you didn't notice, we're now in the 21st century.
In case he has forgotten, let me remind him that he's no longer my boss.
All this is about what is/was/should be done in preparation for a possible situation. As in the examples, 'in case' is usually found in the sense 'because of the possibility of something happening or being true'. It's something done in advance, not after the scenario becomes evident.
So when giving instructions to a babysitter for example, rather than saying 'Give him a toy in case he cries', one should go for something like 'Have a toy ready in case he cries'. Or else, use the plain old 'if' instead of 'in case': 'Give him a toy if he cries.'
Let's take one more example:
Buy some soft drinks in case somebody comes to visit. (Buy them in advance as somebody just might come to visit.)
Buy some soft drinks if somebody comes to visit. (Buy them after the visitor has arrived.)
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