The cat is in bed with a fever.
Perhaps the 2nd most well known pun in Japanese. Guaranteed to bring a pained look and a sigh from all but the youngest of audiences. 
The word for cat (neko), and a word which literally means to be “tucked-in-tight” or simply to stay in bed (nekomu) are the sources for this side splitter. Be careful not to laugh too hard and end up bedridden yourself!