json.Marshal(struct) returns empty JSON "{}": how to fix?

Hi everyone! I'm new to Go and encountered a strange problem when working with JSON. I have a struct, I fill it with data, try to serialize it to JSON using `json.Marshal()`, but the output only gives empty curly braces `{}`. Here's my code: ```go package main import ( "encoding/json" "fmt" ) type User struct { Name string Age int } func main() { u := User{"Alice", 30} result, err := json.Marshal(u) if err != nil { fmt.Println("Error:", err) return } fmt.Println(string(result)) // Outputs: {} } ``` I expected to see `{"Name":"Alice","Age":30}`, but I only see `{}`. There's no error, but the data is lost. What am I doing wrong? I checked that the struct is filled correctly — if I print the fields via `fmt.Println(u.Name, u.Age)`, they