Metal block

Metal Blocks are large, metallic blocks that are found in the Paper Mario series.

These blocks typically serve as barriers which block a path to either a new or hidden area. The only way Mario can destroy the blocks and proceed beyond them is by using the Ultra Hammer, which he finds in Chapter 5 during his expedition into Mt. Lavalava. Luigi will be sitting on one after Mario gets an Ultra Hammer.

In Paper Mario: The Thousand-Year Door, a few Metal Blocks can be found in two rooms of the Riddle Tower. Here, they cannot be destroyed, even with the Ultra Hammer; instead, Mario must smash the corresponding Stone Blocks in the other room to eliminate them.

Tattles
Paper Mario: (Needs to be added)

The Thousand Year Door: "You can't break this block. Seriously. No matter what you do, you can't break this."