> For the complete documentation index, see [llms.txt](https://docs.aeralend.app/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.aeralend.app/how-it-works/lending-and-acook.md).
# Lending and aCOOK
COOK is the only asset you can lend, and the only asset anyone can borrow. bCOOK is collateral and is never lent — that is a program rule, not a presentation choice, which is why bCOOK earns nothing on Aera and keeps earning at BakeYourStake instead.
## aCOOK is your claim on the pool
When you lend COOK you receive **aCOOK**. It does not pay out; its redemption value rises.
The pool has a supply index that grows as borrowers pay interest. Your aCOOK balance never changes — what changes is how much COOK each unit redeems for. Lend 1,000 COOK, and when the index has risen 4.3% you redeem the same 1,000 aCOOK for 1,043 COOK. There is no claim step and no compounding decision.
## What you can do with it
| | |
| ------------------------- | ----------------------------------- |
| Redeem it for COOK | Yes, subject to available liquidity |
| Hold or transfer it | Yes — an ordinary SPL token |
| **Post it as collateral** | **No.** The program refuses it. |
The last one is deliberate. aCOOK is a claim on the same pool you would be borrowing from, so accepting it as collateral would let the pool collateralise loans against itself.
## Getting your COOK back
The limit is whatever is **available** in the pool. If borrowers have taken most of the liquidity you can withdraw only what is free, and you wait for repayments or for new lenders.
This is normal for a lending pool and it is not a fault — but it is a real risk, and worth understanding before lending an amount you may need back at short notice. The [interest curve](/how-it-works/interest-and-fees.md) is designed to resolve it quickly by making borrowing expensive as the pool empties.
{% hint style="info" %}
Locking bCOOK produces **aBCOOK** internally, the same kind of share token for the collateral reserve. You will see it named in transaction details. It is not something you need to hold or manage.
{% endhint %}
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