Solutions to what I'll call Mochel's Conundrum.

The problem:
Given a file, concatonate 2 concurrent lines into 1.

Example:

a b c
d e g
h
i
j k

becomes:
a b c d e g
h i
j k


Solutions:

while read xx
do
	if [ -z $line ]
	then
		line=$xx
	else 
		echo $line $xx
		line=''
	fi
done
if [ $line ]
then
	echo $line
fi

# other shell answer

while read xx
do
	read yy
	echo $xx ${yy}
done < foo.txt

# here's the perl equivalent

open(FILE,"foo.txt");
while(<FILE>) {
	chomp;
	print $_ . " " . <FILE>;
}
print "\n";
close FILE;

# One line answers
sed ':1~2:a;N;s/\n/ /g' foo.txt 
xargs -l2 < foo.txt 

# Inaky's 1 liner
awk '{printf("%s ", $0); getline; print}' foo.txt
Bugfix to the above: awk '{printf("%s ", $0); if (getline) { print } }' foo.txt

# Jesse's short perl 1 liners
perl -pe 'chomp; $_ .= " " . <>' < foo.txt
perl -pe 's/\n/" " . <>/e' < foo.txt