Due to processes the include wind, water, ice, chemicals and gravity rocks break down to smaller pieces. If they remain the same in composition, they probably mechanically weathered. If they changed in composition, they chemically weathered.
Mechanical weathering is the same thing as physical weathering. This means that the rocks are actually broken apart. In this case the rock is exactly the same as it’s bigger sized original, except that it is smaller. Rocks can be physically weathered by Ice wedging or Frost Action. In this case, ice wedging refers to water getting into cracks freezing and expanding. Frost Action is the freezing and warming of water which causes expansion and contraction and eventually leads to breaking. Plant roots can cause mechanical weathering, as can wind. Wind actually can weather by moving sand to almost “sand blast” pieces of a rock off. We see this on monuments and stone statues like the Sphinx. Flowing water from a stream can also weather rock. We call this abrasion. The constant bump and rub against the rock eventually breaks it down to particles which are carried by the stream and lead to further collisions and breaking down of other rocks. Finally, gravity leads to breakage as well. As rocks travel down hill they also bump and break, on the way down. Waves are another type of water abrasion. This constant attack on the rocks eventually wears and breaks rocks down.
Mineral composition is a contributing factor in physical weathering, as harder minerals are difficult to break and softer minerals weather easier. Quartz and Feldspar resist weathering, but often do break. Mica and Calcite break apart easier. Another mineral, Kaolinite (clay) weathers easily as well.