bite, eat
咀 centers on using the mouth to break down food: biting and eating.
咀 combines 口 (mouth) with 且, which likely contributes the sound and may have originally depicted a pile of offerings, but the exact historical development is uncertain.
The mouth 口 is next to 且, which looks like a stack of food. Imagine a mouth biting into a pile of stacked food.
For ソ, picture a person biting into a piece of food and saying 'so' good: so -> ソ.