When dealing with waves in pipes, we consider two different types of pipe.  Closed pipes are closed at one end and only produce odd harmonics (such as a milk bottle - see Experiment 3).  Open pipes are open at both ends and form the basis of most wind and brass instruments.

The reason that closed pipes produce only odd harmonics is due to the fact that a pipe must have an antinode at its open end or ends, but any boundary causes a node.

As shown in the diagram, a closed pipe must always have an antinode at its open end and a node at its closed end.   As you can see the fundamental is 1/4 of a wavelength, which is an odd harmonic.

In an open pipe, there is an antinode at both ends.   This means the shortest wave which can fit inside is half a wavelength.