You're outside on the street with a friend and you see a teenage couple kissing. You don't think it's proper for them to do that in front of strangers, so you say to your friend:
Can you believe these kids just making out on the street?
The phrase "Can you believe ___?" is used to criticize something that you don't like. For example:
Can you believe he didn't even call me back?
Can you believe this shit?
Use this when you think people are doing something that's totally unacceptable, and you're surprised that they're doing it. You usually say this to someone who you think agrees with your opinion.
"Just (did something)" is often used along with this phrase:
Can you believe she just took it without asking? How rude!
If you use the word "kids" to describe teenagers, it sounds like you don't respect them. In this case, the speaker doesn't respect these teenagers, so it's correct!
"Making out" means kissing heavily. It's kind of a slang expression, but it's been used since the 1950's, so it's widely understood by people of all ages.
When you're specifically describing the location of something, "on the street" means that something is on the part of the street that cars drive on. But "(doing something) on the street" just means that you're doing something outside in a city.
"On the street" is sometimes associated with bad behavior and crimes:
There's people on the street fighting, selling drugs, and God knows what else.
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