In Excel, you can interact with user input through VBA (Visual Basic for Applications) code. VBA allows you to create custom functions and functionalities within Excel. Here's how to use message boxes and input forms for user interaction:

1. Message Boxes:

- Use the `MsgBox` function to display messages and capture user choices.

```vba
' Display a message with OK and Cancel buttons
result = MsgBox("Are you sure you want to continue?", vbYesNo)

' Check the user's choice
If result = vbYes Then
' Perform actions if user clicks Yes
Else
' Perform actions if user clicks Cancel
End If
```

2. Input Forms:

- Use the `InputBox` function to display a dialog box where users can enter text.

```vba
' Prompt user to enter a name
name = InputBox("Enter your name:", "User Input")

' Check if user entered a name
If name <> "" Then
' Use the entered name in your code
Else
' Handle case where user clicks Cancel or enters nothing
End If
```

Additional Options:

- You can customize message boxes with different icons, buttons, and titles.
- Input forms can be expanded to accept numbers, dates, or even formulas.
- For more complex user interaction, you can create custom user forms with various input fields and buttons using the VBA UserForm editor.

Here are some resources to learn more about VBA in Excel:

- Microsoft Documentation on InputBox: [https://learn.microsoft.com/en-us/office/vba/language/reference/user-interface-help/inputbox-function](https://learn.microsoft.com/en-us/office/vba/language/reference/user-interface-help/inputbox-function)
- Microsoft Documentation on MsgBox: [https://learn.microsoft.com/en-us/office/vba/language/reference/user-interface-help/msgbox-function](https://learn.microsoft.com/en-us/office/vba/language/reference/user-interface-help/msgbox-function)
- Introduction to VBA for Excel: [https://learn.microsoft.com/en-us/office/vba/library-reference/concepts/getting-started-with-vba-in-office](https://learn.microsoft.com/en-us/office/vba/library-reference/concepts/getting-started-with-vba-in-office)

Remember: Using VBA requires some programming knowledge. If you're new to VBA, consider starting with the resources above and practice with simple examples before implementing them in your spreadsheets.