[xstrae]

Hi, can someone explain why this is so?

When some sand is dropped into a beaker, it settles down and spreads evenly at the bottom of the beaker after a couple of hours. When it is picked up, the sand particles stick or clump together. Why is this so?

[Bakegaku]

It's probably because of currents caused in the water (I'm assuming you meant a water-filled beaker  ).  Where small currents are moving downward, it spread the sand out from that area, and the sand is sucked into smaller currents that are moving upwards but is too heavy to be actually lifted so it makes a pile there.