Translation:The sun was so hot, that we saw the butter melt.
A wonderful example of fandi. To start solid and then to melt into a (potentially) new shape. As opposed to deĝeli, which is more what ice does.
Of course, for it to be truly hot, the butter needs to melt on the sidewalk and then cook an omelet.
Why use the infinitive in that last word? It seems like that second half of the sentence reads, "...we saw the butter to melt". Would something like "ni vidis la butero fandigxanta" be correct?
First, it would be “fandiĝantan”, not “fandiĝanta,” as it's not the seeing that melts the butter.
Second, why are you putting a “to” in your translation? “melti” means both “to melt” and “melt,” depending on the English context.
My physics professors refused to accept my citations from They Might Be Giants, unfortunately:-(