黏土 (niántǔ): clay

黏 (nián) means 'sticky / adhesive'. It's often written 粘土, since 粘 also reads nián for the adjective sense. Both 黏土 and 粘土 are accepted; mainland textbooks have shifted toward 黏 since the 2013 General Standard Chinese Character Table fixed it as the standard form.

memory

Think of it as 'sticky earth' — 黏 'sticky' + 土 'earth/soil'. That's exactly what makes clay clay: fine particles that stick when wet.