Question

what is the difference between a pebble and a boulder?
a boulder is lighter
a pebble is smaller
a pebble is heavier

Explanation:

In geology (a subfield of Natural Science), the key distinction between a pebble and a boulder is size. Pebbles are small (typically 4-64 mm), while boulders are large (over 256 mm). Weight varies by material, but size is the defining difference.

Answer:

a pebble is smaller