SB 8.6.20

arayo ’pi hi sandheyāḥ
 sati kāryārtha-gaurave
ahi-mūṣikavad devā
 hy arthasya padavīṁ gataiḥ
Word for word: 
arayaḥ — enemies; api — although; hi — indeed; sandheyāḥ — eligible for a truce; sati — being so; kārya-artha-gaurave — in the matter of an important duty; ahi — snake; mūṣika — mouse; vat — like; devāḥ — O demigods; hi — indeed; arthasya — of interest; padavīm — position; gataiḥ — so being.
Translation: 
O demigods, fulfilling one’s own interests is so important that one may even have to make a truce with one’s enemies. For the sake of one’s self-interest, one has to act according to the logic of the snake and the mouse.
Purport: 

A snake and a mouse were once caught in a basket. Now, since the mouse is food for the snake, this was a good opportunity for the snake. However, since both of them were caught in the basket, even if the snake ate the mouse, the snake would not be able to get out. Therefore, the snake thought it wise to make a truce with the mouse and ask the mouse to make a hole in the basket so that both of them could get out. The snake’s intention was that after the mouse made the hole, the snake would eat the mouse and escape from the basket through the hole. This is called the logic of the snake and the mouse.