1. Explain it simply
Subject + 怎么 + predicate asks 'how come?', showing that something surprises you.
It sits right before the verb or adjective, not at the end of the sentence.
It is more emotional than the neutral 为什么 ('why').
2. Show examples
你怎么不吃了？
Nǐ zěnme bù chī le?
How come you're not eating anymore?
surprise
今天怎么这么冷？
Jīntiān zěnme zhème lěng?
How come it's so cold today?
astonishment
你怎么来了？
Nǐ zěnme lái le?
How come you came?
unexpected
3. Share a tip
怎么 here asks 'how come?' with surprise - different from neutral 为什么, and different from 怎么样 ('how/what about').