Conditional Tooltip Based On Checkbox WPF

I've got a checkbox and a button in my WPF MVVM app. If the checkbox is checked I want a tooltip that says "x" and if it's unchecked the tooltip should say "y".

Anyone know the best way to do this? I think it could be done with a separate property in my view model, but maybe there's an easier way to do this in just the xaml?

Thanks in advance.