How can I distinguish floating versus sinking LEGO pieces? My kids enjoy playing with their LEGOs in the pool, and we cannot always predict which ones will float and which one will sink.

It appears to me that most pieces do float in the pool, but some, for instance a motorbike, doesn't.

How can I distinguish, if at all possible, which ones will float, before actually putting them in the pool?
Most Lego pieces sink. Some of them will float briefly if they have air trapped inside. Almost any construction of Lego elements will sink as there are gaps between pieces.

There are some boat pieces that are designed to float, such as the Police Patrol Boat set from 2016. A Brickset list shows some other boats that float.