Retirement tax questions

So let's do a hypthetical... 

Let's say I fall into the > 25K theshold, i.e. if I were to get $50,000 in Social Security (which I am not, it's just an example).

 

So half of SS income (50K - 25K) is 25K and that hit's the threshold. What confuses me is the amount I would pay tax on? Is the entire $50K taxable because it exceeds the limit? Or is JUST the amount that goes over the limit taxable. In this case will I be paying tax on 50K or 25K ???  

 

In other words, if you go over the 25K threshold, is the first 25K being taxed, or JUST the amount over 25K.