steaming rice
餾 centers on the specific cooking process of steaming rice, using heat and moisture to prepare the grain.
餾 combines 飠 (food) with 留, which likely contributes the sound and may relate to the idea of retaining or holding, as in steaming rice in a closed vessel.
The food radical 飠 on the left and 留 (to detain, keep) on the right suggest keeping food over steam: rice is held in a steamer until it's cooked.
For リュウ, imagine a steaming pot of rice releasing a long, steady 'ryuuuu' sound as the steam escapes: ryuu -> リュウ.
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