Creating a rule for a file type

Question

How can I create a rule in a makefile for a specific filetype so if I write something like

result.txt: first.txt second.txt

in a makefile and it will concatenate prerequisites.

.txt:
     cat $? > $@

doesn't work

Answer 1

Since this seems to have helped, I am reposting it as an answer, with @eriktous' comment added in.

%.txt:
        cat $? >$@

If you don't want to process just the changed files, use $^ instead of $?.