The trick is that the height of each line defaults to 1em. Setting the max height and overflow hidden will only show the first line of the sentence.
p {
max-height: 1em;
overflow: hidden;
}
Another approach could be to use text-overflow: ellipsis. This is much more user-friendly as it indicates that the sentence has more text and is trimmed.
http://jsfiddle.net/Wws6L/11/
p {
overflow: hidden;
text-overflow: ellipsis;
white-space: nowrap;
width: 100%;
}