Tomas Pearson in Stockholm asked:
Why butter is so hard to spread on newly baked bread, whereas peanut butter is so easy? Butter tends to form clumps whereas peanut butter spreads out as it would with cold bread.
Peanut butter is rich in vegetable oil and that means it has unsaturated fats in it. An unsaturated fat has double bonds between the carbon atoms, which cause the hydrocarbon chain to bend. This bending causes the molecules not to stack together tightly, which means that the peanut butter is less dense and therefore will spread more easily than something that contains saturated fats.
Saturated fats have single bonds between the carbon atom so they stack together more closely and are very dense. So because butter is more dense, it's harder to spread than peanut butter. This means that butter forms globules and sticks to itself rather than spreading out evenly on the bread.
It might also be that if the bread has just come out of the oven, it's still warm and the melting point of unsaturated fats is lower than saturated fats, so the peanut butter (unsaturated fat) would get runnier (start to melt) quicker and becomes easier to spread.