(same with tahini)
In: 1
You’re initially forming an emulsion (which is thick, like mayonnaise).
Peanut butter and tahini are a bunch of solid seed bits (peanut or sesame) in oil. Plus, usually, a bunch of emulsifiers to keep everything nicely suspended and prevent separation.
When you introduce water into that and start stirring the emulsifiers grab it and make an emulsion (a suspension of small water droplets in oil). This thickens the fluid phase just like how mayonnaise (water-in-oil) or salad dressing (oil-in-water) is thicker than either water or oil alone. Combined with all the solids, that makes things really thick locally where the emulsion formed.
As you mix it in more the the emulsion spreads out (and may break), and the extra volume of water you’re adding eventually dilutes everything down and back to the normal thin consistency you were expecting.
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