$M,N,O,P$ and $Q$ are five friends. Each of them has a different height. $O$ is taller than $P$ and $N$ and shorter than $M$ and $Q$ only. Only one person is shorter than $N$. Who is the shortest?

Let's arrange the five friends in descending order of their heights ($1^{\text{st}}$ being the tallest, $5^{\text{th}}$ being the shortest):
1. "O is shorter than M and Q only": This means exactly two people ($M$ and $Q$) are taller than $O$. Therefore, $O$ must be at the exact $3^{\text{rd}}$ position. The $1^{\text{st}}$ and $2^{\text{nd}}$ positions are occupied by $M$ and $Q$ (in any order).

$$\text{(M, Q) > O > \_ > \_}$$

2. "O is taller than P and N": This confirms that $P$ and $N$ must occupy the remaining $4^{\text{th}}$ and $5^{\text{th}}$ positions below $O$.
3. "Only one person is shorter than N": This implies that $N$ is at the $4^{\text{th}}$ position, leaving exactly one spot beneath him.
4. Consequently, $P$ must occupy the final $5^{\text{th}}$ position.

The complete height order is:

$$\text{(M / Q) > (Q / M) > O > N > P}$$

Thus, $P$ is at the lowest position, making him the shortest among them.