Some urban wisdom from Mexico

Context: Using common signs to practice modals verbs of permission, advice and prohibition (must, can, should...)
Me: 'Where can you see this sign and what does it mean?'...
Mexican student: 'My granny used to say that you always kept her valuables safe between the twins...'
Other students: 'the twins?'
Mexican student: 'yeah, the twins...' 
To which he gets hold of his moobs, simulating a cleavage meant to reflect his gran's breasts.. the twins! Better than a safe and no risk of losing the key or forgetting the code.
Conclusion of the lesson: When on public transport, you should keep your valuables between the twins. 
Need to find out what 'stick it between the twins' is in Spanish...